Low-energy Hydrogen Ion Implantation Research Articles

Hydrogen passivation effect on the conversion efficiency of Si solar cells by low-energy proton implantation

We investigated the hydrogen passivation effect on the conversion efficiency of Si solar cells by including low-energy hydrogen ion implantation in the cell fabrication process. The solar cells were implanted with various proton energies and doses just before the co-firing process. The quantum efficiency specifically in the blue wavelengths of the solar cells increased with a low implantation dose and energy indicating improvement of surface passivation. However, quantum efficiency decreased when the implantation dose and energy were increased, indicating a high defect-assisted surface recombination of minority carriers. The cells implanted with a low dose and energy in the presence of SiNx showed interfacial passivation with enhanced photocurrent, minority carrier lifetime and short-circuit current density for improved conversion efficiency.

Effects of low-energy hydrogen ion implantation on optical properties of ZnO nanowires

The low-energy hydrogen ions (2 keV; 1 × 10 15 to 1 × 10 18 cm −2 per dose) implantation was used to study the passivation effect of defects and photoluminescence properties of ZnO nanowires. The implanted H + effectively passivated deep level native defects, making the visible emission at 500 nm disappear completely and the UV emission (380 nm) of nanowires enhance for seven times. H + implantation at higher dose induced a strongly new violet emission broad peak (around 410 nm) which may originate from the hydrogen related complex of defects. However, this violet emission disappeared after annealing in argon atmosphere at 300 °C, confirming that the new violet emission is related to hydrogen. All emission peaks vanished due to the formation of a large quantity of nonradiative recombination centers at high dose implantation. This controllable method of hydrogen doping may find potential application in UV/violet optoelectronic and especially in nano-optoelectronic devices.

Ion beam analysis of hydrogen retained in carbon nanotubes and carbon films

Carbon nanotubes (CNTs) are studied as a possible hydrogen storage medium for future energy needs. Typically, hydrogen is stored in the CNTs by exposure of the material to a high-pressure H2 atmosphere at different temperatures. The maximum hydrogen concentrations stored following this method and measured using ion beam analysis do not exceed 1wt.%. Introduction of defects by ion irradiation (i.e. implantation) prior to high-pressure H2 treatment, offers an alternative method to activate H adsorption and enhance the chemisorption of hydrogen. This is a preliminary work where hydrogen was introduced into single-wall nanotubes and carbon films by low-energy (13.6keV) hydrogen ion implantation. Elastic recoil detection was used to measure the quantity and depth distribution of hydrogen retained in the carbonaceous materials. Results show that there are substantial differences in the measured profiles between the CNT samples and the vitreous carbon. On another hand, only ∼43% of the implanted hydrogen in the CNTs is retained in the region where it should be located according to the SRIM simulations for a solid carbon sample.

Modification of electrical conductivity in RF magnetron sputtered ZnO films by low-energy hydrogen ion implantation

Abstract The electrical conductivity of ZnO films deposited on Si by the RF sputtering technique were modified by low-energy hydrogen implantation. Hydrogen ions were implanted at 25 keV into various ZnO films with doses ranging from 1 × 1016 to 1.0 × 1017 ions cm−2. High resolution quantitative hydrogen depth profiles of the un-doped and doped samples were measured non-destructively with elastic recoil detection analysis using a finely focussed 2.5 MeV4He+ ion beam impinging onto the samples at 20°. Structural and electrical measurements were carried out on the films by SEM, XRD and Hall probe measurements to explore the effects of hydrogen implantation. It was observed that the electrical conductivity is decreased by as much as an order of magnitude after hydrogen implantation. The films remained polycrystalline after implantation. The results suggest that reducing the n-type conductivity using low-energy hydrogen implantation without annealing may provide a pathway for passivating the donors for a possible subsequent formation of p-type acceptors by ion implantation.

DLTS of low-energy hydrogen ion implanted n-Si

We have used deep level transient spectroscopy and capacitance–voltage measurements to study the influence of low-energy hydrogen ion implantation on the creation of defects in n-Si. In particular, we have studied the ion fluence dependence of the free carrier compensation at room temperature, and we have measured the generation of VO–H complex and VP-pair in ion implanted samples. The 7.5keV H ions created defects in the top 0.3μm of samples, which resulted in carrier compensation to depths exceeding 1μm. This effect is not due to defects created by ion channeling but is rather due to the migration of defects as demonstrated using binary collision code MARLOWE.

X-ray-emission study of the structure of Si:H layers formed by low-energy hydrogen-ion implantation

Amorphous silicon layers formed by implantation of 24-keV hydrogen ions into SiO2/Si and Si with doses of 2.7×1017 and 2.1×1017 cm−2, respectively, were studied using ultrasoft X-ray emission spectroscopy with variations in the energy of excitation electrons. It is ascertained that the surface silicon layer with a thickness as large as 150–200 nm is amorphized as a result of implantation. Implantation of hydrogen ions into silicon coated with an oxide layer brings about the formation of a hydrogenated silicon layer, which is highly stable thermally.

Chemical defect explanation for the effect of postdeposition treatments on CuInSe2

Hydrogenation and oxygenation of CuInSe2 have been studied by x-ray photoelectron spectroscopy in order to investigate the defect properties. Initially oxidized p-type material was type-converted by low-energy hydrogen ion implantation. A defect model has been proposed suggesting the reactivation of selenium vacancies (VSe) as well as the creation of additional indium copper antisites (InCu). For cleaved samples, a direct influence of hydrogen on the net-donor concentration has been assumed. The resulting Cu-depleted surface is the result of the downward band bending and the migration of Cu into the bulk by the built-in electric field. The subsequent oxidation leads to a passivation of VSe and InCu, and hence yields a reconversion of the conductivity type.

Conductivity type conversion of p-type CuInSe 2 due to hydrogenation

The surface of p-type CuInSe 2 single crystals prior to and after low energy hydrogen ion implantation at increased substrate temperature has been investigated by X-ray photoelectron spectroscopy (XPS) and Raman measurements. A shift of the Fermi-level by 500 meV closer to the conduction band after the hydrogen exposure has been observed for CuInSe 2, oxidized by storage in air. The same behavior was observed for a clean surface prepared in ultra-high vacuum (UHV) by cleavage of CuInSe 2. This observation supports the previously suggested type conversion from p- to n-type of the surface of CuInSe 2. The conversion is explained by the passivation of Cu vacancies and hydrogen on and interstitial site. For the oxidized sample, the re-activation of oxygen passivated Se vacancies might influence the type conversion additionally. Raman spectroscopy did not reveal any lattice damage after implantation at 300°C sample temperature.

Surface Quantum Wells in Hydrogen Implanted ZnO

Low energy hydrogen ion implantation in ZnO creates an accumulation layer near the surface, giving rise to a quantum well. The corresponding self-consistent Hartree problem is solved by taking into account the donor distribution resulting from the implantation process. The values of the surface potential as a function of repelled away electron concentration are derived. The resulting data are compared with experimental values obtained from space-charge capacitance measurements. A comparative study between the semi-classical and self-consistent quantum treatments is presented.

The study of structural properties of 100 keV hydrogen ion implanted semi-insulating GaAs single crystals

100 keV hydrogen ion implantation has been carried out on undoped semi-insulating 〈1 0 0〉 gallium arsenide single crystals for various ion doses at room temperature. The structural properties due to high dose, low energy hydrogen ion implantation has been investigated using X-ray double crystal diffractometry (DCD) analysis, Rutherford backscattering spectrometry and channeling (RBS/C) experiments and transmission electron microscopy (TEM) analysis. By using DCD analysis, the value of elastic lattice strain for the ion doses of 1 × 10 16, 1 × 10 18 ions/cm 2 has been estimated to be 2.1 × 10 −3 and 3.2 × 10 -3, respectively. The RBS spectra in the channeling mode for the high dose implantations (10 17 and 10 18 ions/cm 2) show a high yield indicating a highly damaged region near the range of the implanted hydrogen ions. Particularly, for the dose 10 18 ions/cm 2, a heavily damaged region at the surface can be observed. The TEM results evidenced that no amorphisation occurred for the dose 10 18 ions/cm 2. From TEM characterisation, it is also observed that there are no hydrogen bubbles present in the implanted region. But small dislocation loops have been identified. The projected range of implanted hydrogen ions and the thickness of the implanted layer obtained by RBS and TEM analysis are compared with the TRIM calculations.

Hydrogen passivation of multicrystalline silicon solar cells

The use of hydrogen for passivation of multicrystalline silicon in solar cell technology is described. Three kinds of hydrogen incorporation into mc-Si solar cells have been evaluated: hydrogen diffusion out of a SiN-layer (SiN:H), low-energy hydrogen ion implantation (HII), and remote plasma hydrogen passivation (RPHP). Best results were obtained by RPHP, whereas using HII, damage exceeded the passivation effect to some extent. While SiN:H passivates more the grains, RPHP acts particular on grain boundaries. Combining hydrogen passivation by SiN:H and RPHP leads to optimal bulk passivation. We have investigated the influence of RPHP to various mc-Si materials by solar cell and diffusion length measurements. CVD layers and ribbon material show the strongest increase in performance. A boost of up to 77.0 mV in the open-circuit voltage and 2.0% in the efficiency of solar cells has been achieved. Electromagnetically casted Si has shown an improvement of 2.1% in efficiency after RPHP treatment. But even mc-Si of higher quality, like mc-Baysix and Eurosolare mc-Si could be rectified by RPHP. Applying a standard cell process for material assessment efficiencies of 16.9% have been obtained including a RPHP step. Thus the potential for high-efficiency mc-Si solar cells is shown.

A modified broad beam ion source for low-energy hydrogen implantation

A modified broad beam ion source for low-energy hydrogen ion implantation of semiconductors is described. Based on a Kaufman type ion source two different solutions are presented: (a) an ion source with an extraction system consisting of two molybdenum grids with a low gas flow conductance reworked for hydrogen operation, and (b) a ten-grid mass separating ion beam system which enables the mass selection of H+, H2+, and H3+. The ion energy could be set in the range of 200–500 eV with a current density reaching from 1 to 100 μA/cm2. It is shown that at higher pressure the main ion created in the ion source is H3+ due to ion-molecule processes, whereas at lower pressure only H2+ and H+ are produced. Special consideration is given to the ion beam analysis of the two grid ion source operating in the 10−3 mbar range allowing to explain the different peak structures by the potential distribution across the ion source and different charge transfer processes. In addition, the analysis reveals neutral and ionized ...

Enhanced x-ray optical contrast of Mo/Si multilayers by H implantation of Si

To increase the x-ray optical contrast of Mo/Si multilayers, we study low energy hydrogen ion implantation of amorphous Si layers. Using elastic recoil detection and Rutherford backscattering spectrometry, we measure the result of hydrogen implantation on Si atomic density. We find a lowering of Si atomic density, and, thus, an enhancement of x-ray optical contrast, as a result of H implantation. We find that the Si atomic density saturates at a minimum of 64±5% of the crystalline value. We have also observed a minor smoothing effect of H+ ion bombardment. Combined with Kr+ ion bombardment, causing a very much larger smoothing of the Si surface, the atomic density is found to saturate at a minimum of 77±5% of the crystalline value.

Hydrogen Passivation of Defects in Electron Irradiated Polycrystalline Silicon Solar Cells

AbstractThe effect of hydrogen passivation on photovoltaic performance of 1 MeV electron irradiated polycrystalline cast silicon solar cells is described. These cells were processed on cast p-type boron doped polycrystalline silicon substrates using standard technology. Passivation was made by low-energy hydrogen ion implantation on the front side. Cells performance was measured as a function of fluence, and it was found that the hydrogenated cell had the higher radiation resistance.Defect behavior were studied using deep level transient spectroscopy and infra-red spectroscopy. It was shown that the concentration of vacancies (Ec −0,09 eV), divacancies (Ec −0,23 eV) and A-centers (Ec −0,18 eV) is significantly lower in hydrogenated samples. This consistency strengthens the belief that hydrogen interacts with vacancy-type defects to prevent formation of the secondary radiation defects. It is confirmed by IR-measurements.

Rapid Thermal Process-Induced Defects : Gettering of Internal Contaminants

AbstractWe show in this study that RTP-induced defects analysed by Deep Level Transient Spectroscopy (DLTS) are related to residual impurities present in as-grown silicon wafers. For one particular material an activation of a specific residual metallic impurity was observed in the temperature range 800 - 1000°C. This impurity can be returned to an electrically inactive precipitated form by classical thermal annealing (CTA) with a slow cooling rate or neutralized by means of low-energy hydrogen ion implantation.

Passivation of polycrystalline silicon solar cells by low-energy hydrogen ion implantation

It has been demonstrated that hydrogen can be used to saturate dangling bonds in amorphous silicon, so this element has been chosen to passivate the defects present in polycrystalline materials. In recent years, it has been recognized that hydrogen ion implantation at low energy also results in a great improvement in the electrical properties of polycrystalline silicon solar cells. Technically, the best approach is to use a Kaufman-type ion source or similar equipment in order to reduce the processing time. We have used a multiple-beam ion source based on the same principle as the Kaufman-type source, but equipped with a post-acceleration electrode providing H + ions with 0.5–10 keV energies. For this equipment, we have determined the following: (1) the effective distribution and concentration profiles of the introduced hydrogen and the modification of the optical properties; (2) the conditions under which two cast multicrystalline materials (Polyx and Silso) will give the greatest improvement in cell performance; (3) the long-term electrical stability of solar cells processed by the methods described. This work provides the first consistent picture of the relationship between hydrogen-ion-beam parameters and optimal and electrical properties.

Inert two-dimensional accumulation layers at the free ZnO surface produced by hydrogen implantation

Low-energy hydrogen-ion implantation is use to produced extremely strong electron accumulation layers on ZnO. Such layers are 10–20 Å wide and resemble closely accumulation layers obtained by conventional methods. They are, however, completely insensitive to oxygen and thus constitute the only two-dimensional electron gas system on a surface that is both free and inert.

Passivation of laser induced defects in silicon by low energy hydrogen ion implantation

It is well established that the pulsed-laser annealing of as-implanted Si introduces electrically active defects. The aim of this paper is to show that these defects can be neutralized by low-energy hydrogen ion implantation. The techniques used for the sample characterization are current-voltage measurements and capacitance transient spectroscopy.

Penetration depths of low-energy hydrogen-ion implantation on ZnO surfaces

The penetration depths of protons in the (0001̄) face of ZnO produced by 100- and 400-eV hydrogen-ion bombardment are studied by three methods: calibrated argon-ion sputtering, calibrated etching, and space-charge capacitance measurements in the ZnO/electrolyte system. For the 100-eV implantation, the latter method provides unequivocal support to our previous conclusion that the protons, acting as fully ionized donors, penetrate only to a depth of 10–20 Å below the surface. The narrow space-charge layer so produced, having surface electron densities of up to 2 × 1014 cm−2, constitutes a quantized, two-dimensional electron gas system. In the 400-eV implanted surface, the proton penetration depth is considerably larger. In this case an approximate depth profile could be derived from the combined measurements. It consists of a Gaussian distribution, peaked about 40 Å below the surface with a standard deviation of some 80 Å. However, some 10% of the implanted protons are found to penetrate much deeper, being distributed up to 1000 Å or more below the surface. For 100-eV implanted surfaces, similarly large penetration depths were observed, but the percentage of the deep lying protons is less than 1%. Such huge penetration depths arise most probably from a channeling mechanism.

Low energy hydrogen-ion implantation on thermally treated ZnO surfaces

Studies are reported of ZnO surfaces prepared by a three-stage implantation procedure consisting of: (1) bombardment with a 100-eV hydrogen-ion beam of an insulating sample held at 200°C, a process that introduces a very high surface density of free electrons; (2) annealing at 300°C, which restores the initial insulating state of the surface, suggesting loss of all memory of the preceding implantation; and, finally, (3) re-bombardment with 100-eV ions, but now while the sample is kept at room temperature. The surfaces so produced are totally different from those reported previously in which only the last implantation stage had been applied. In addition to the narrow, electron-rich layer resembling very closely the accumulation-like layer observed on the latter surfaces, a second, much wider electron-rich layer also forms. The first layer, extending to about 20 Å below the surface proper, contains hydrogen donors which are fully ionized down to at least 10 K. The second layer is several thousand angstroms wide and the hydrogen donors in it are only partially ionized at room temperature, becoming completely unionized at 100 K. The enormous penetration range of the implanted hydrogen, as evidenced by the thickness of the wide layer, is very difficult to account for. Equally difficult to explain is the marked difference in the characteristics of the hydrogen donors in the narrow and wide layers. Some suggestions concerning these rather puzzling observations are put forward but at this stage they are largely of a speculative nature.