Dynamics of the ion beam induced nitridation of silicon

High-resolution Rutherford backscattering and channeling has been used to study the energy and angular dependence of the ion beam induced nitridation of Si in a secondary ion mass spectrometry system. The nitridation of Si is characterized by two critical angles θc1 and θc2, corresponding to the formation of stoichiometric and overstoichiometric Si-nitride layers, respectively. For the N₂⁺ bombardment in the 10 to 13.5 keV range, θc1 changes from 40° to 45°, while θc2 changes from 28° to 30°. Further, strong oscillations in the secondary ion signal, observed for angles of incidence below θc2, are directly related to charging of the Si-nitride surface. We demonstrate that the response of the Si-nitride layer under ion bombardment during the transient stage of nitridation can be described by a second order differential equation

Electrical transients in the ion-beam-induced nitridation of silicon

We have studied the dynamics of the initial stages of silicon nitride formation on siliconsurfaces under nitrogen beam bombardment in the secondary ion mass spectrometry apparatus. We have shown that the secondary ion signal exhibits damped oscillations below the critical impact angle for nitride formation. We have described this oscillatory response by a second-order differential equation and argued that it is initiated by some fluctuations in film thickness followed by the fluctuations in surface charging

Sliver Solar Cells

Sliver solar cells are thin, mono-crystalline silicon solar cells, fabricated using micro-machining techniques combined with standard solar cell fabrication technology. Sliver solar modules can be efficient, low cost, bifacial, transparent, flexible, shadow-tolerant, and lightweight. Sliver modules require only 5 to 10% of the pure silicon and less than 5% of the wafer starts per MWp of factory output when compared with conventional photovoltaic modules. At ANU, we have produced 20% efficient Sliver solar cells using a robust, optimised cell fabrication process described in this paper. We have devised a rapid, reliable and simple method for extracting Sliver cells from a Sliver wafer, and methods for assembling modularised Sliver cell sub-modules. The method for forming these Sliver sub-modules, along with a low-cost method for rapidly forming reliable electrical interconnections, are presented. Using the sub-module approach, we describe low-cost methods for assembling and encapsulating Sliver cells into a range of module designs

Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies

We have investigated compositional changes on GaNsurfaces under Ar-ion bombardment using synchrotron-based high-resolution x-rayphotoemission (PES) and near-edge x-ray absorption fine structure(NEXAFS)spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemissionspectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-rayabsorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaNsurfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.P.N.K.D. is grateful for the financial support of the Australian Research Council

Onset of implant-related recombination in self-ion implanted and annealed crystalline silicon

The impact of residual recombination centers after low-energy self-implantation of crystalline silicon wafers and annealing at 900 °C has been determined by bulk carrier lifetime measurements as a function of implant dose. Doses below 10¹³cm⁻² resulted in no measurable increase in recombination, while higher doses caused a linear increase in the recombination center density. This threshold value corresponds to the known critical dose required for the formation of relatively stable dislocation loops during high temperature annealing.Deep level transient spectroscopy revealed a decrease in the vacancy-related defect concentration in the high-dose samples, which we interpret as reflecting an increase in the siliconinterstitial concentration. This suggests that siliconinterstitials, arising from the slowly dissolving dislocation loops, may be responsible for the increased recombination deep within the samples.This work has been supported by the Australian Research Council

Implant isolation of Zn-doped GaAs epilayers: effects of ion species, doping concentration, and implantation temperature

The electrical isolation of Zn-doped GaAs layers grown by metalorganic chemical vapor deposition was studied using H, Li, C, and O ion implantation. The ion mass did not play a significant role in the stability of isolation, and a similar activation energy of ∼(0.63±0.03 eV) was obtained for isolation using either H or O ions. Furthermore, the isolation was stable against isochronal annealing up to 550 °C as long as the ion dose was 2–3.5 times the threshold dose for complete isolation, Dth, for the respective ion species. By studying the thermal stability and the temperature dependence of isolation, we have demonstrated the various stages leading to the production of stable isolation with the increasing dose of 2 MeV C ions. For ion doses less than 0.5Dth,point defects which are stable below 250 °C are responsible for the degradation of hole mobility and hole trapping. The stability of isolation is increased to ∼400 °C for a dose Dth due to the creation of defect pairs. Furthermore, the hopping conduction mechanism is already present in the damaged epilayer implanted to Dth. Higher order defect clusters or complexes, such as the arsenic antisite, AsGa, are responsible for the thermal stability of implantation isolation at 550 °C. The substrate temperature (−196–200 °C) does not have an effect on the isolation process further revealing that the stability of isolation is related to defect clusters and not point-like defects. An average number of eight carbon ions with energy of 2 MeV are required to compensate 100 holes, which provides a general guideline for choosing the ion dose required for the isolation of a GaAs layer doped with a known Zn concentration. A discussion of the results on the implantation isolation of p-GaAs previously reported in the literature is also included.P.N.K.D. acknowledges the Australian Research Council for financial support

Towards a better understanding of the operative mechanisms underlying impurity-free disordering of GaAs: Effect of stress

The effect of stress on defect creation and diffusion during impurity-free disordering of SiOₓ-capped n-GaAs epitaxial layers has been investigated using deep level transient spectroscopy. The oxygen content in the SiOₓ layer and the nature of the stress that it imposes on the GaAs layer were varied by changing the nitrous oxideflow rate, N, during plasma-enhanced chemical vapor deposition of the capping layer. The peak intensity of defects S1 and S4 increased with the increasing nitrous oxideflow rate to exhibit a maximum in the range 80 sccm350 sccm. On the other hand, the peak intensity of S2* increased linearly with N. We have explained the maximum in the intensity of defects S1 and S4 for 80 sccm<N<200 sccm to be due to a corresponding maximum in the compressive stress which is experienced by the capped GaAs layer during annealing. Although the creation of S2*, which we have proposed to be a complex involving the galliumvacancy(VGa), is enhanced with the increasing compressive stress, it also becomes efficiently converted into the arsenic-antisite, AsGa. The compound effect of these opposing mechanisms results in a linear dependence of the peak intensity of S2* on N. This study is to the best of our knowledge the first to provide the evidence for the stress-dependent anti-correlation between VGa- and AsGa-related defects in GaAs. We have also narrowed the origin of S1 to complexes involving arsenic interstitials, Asi, and/or AsGa.P. N. K. Deenapanray and H. H. Tan gratefully acknowledge the financial support of the Australian Research Council

Investigation of reactive ion etching of dielectrics and Si in CHF₃/O₂ or CHF₃/Ar for photovoltaic applications

Using a combination of etch rate, photoconductance, and deep level transient spectroscopy(DLTS) measurements, the authors have investigated the use of reactive ion etching (RIE) of dielectrics and Si in CHF₃∕O₂ and CHF₃∕Arplasmas for photovoltaic applications. The radio frequency power (rf-power) and gas flow rate dependencies have shown that the addition of either O₂ or Ar to CHF₃ can be used effectively to change the etch selectivity between SiO₂ and Si₃N₄. The effective carrier lifetime of samples degraded upon exposure to a CHF₃-based plasma, reflecting the introduction of recombination centers in the near-surface region. The extent of minority carrier lifetime degradation was similar in both types of plasmas, suggesting that the same defects were responsible for the increased recombination. However, the rf-power dependence of lifetime degradation in n- and p-type Si was different. Moreover, the lifetime degradation did not exhibit a linear rf-power dependence, suggesting that primary defects were not the dominant recombination centers responsible for the decrease in lifetime. Indeed, DLTS measurements have shown that secondary defects were formed in samples exposed to the plasma after annealing at 400°C, the temperature at which a SiN:H layer is deposited on samples to passivate their surfaces. The minority carrier lifetime degradation in RIE processed samples could be partially avoided using post-RIE chemical treatments.The authors would like to thank the Australian Research Council for financial support

Formation rates of iron-acceptor pairs in crystalline silicon

The characteristic association time constant describing the formation of iron-acceptor pairs in crystalline silicon has been measured for samples of various p-type dopant concentrations and species (B, Ga, and In) near room temperature. The results show that the dopant species has no impact on the pairing kinetics, suggesting that the pairing process is entirely limited by iron diffusion. This conclusion was corroborated by measurement of the activation energy of pair formation, which coincides with the migration enthalpy of interstitial iron in silicon. The results also indicate that the pair-formation process occurs approximately twice as fast as predicted by a commonly used expression.This work has been supported by the Australian Research Council and the State of Lower Saxony

Impurity-free disordering mechanisms in GaAs-based structures using doped spin-on silica layers

We have used photoluminescence,deep level transient spectroscopy and x-ray photoelectron spectroscopy to investigate the mechanisms of impurity-free disordering in GaAs-based structures using doped spin-on silica layers. We demonstrate that VGa is efficiently converted into arsenic-antisite, AsGa, related defects (EL2-type defects) when the GaAs layer is under compressive stress. We propose that the efficient formation of EL2-type defects reduces the efficiency of impurity-free interdiffusion of GaAs/AlGaAs quantum wells.Two of the authors (P.N.K.D. and H.H.T.) gratefully acknowledge financial support by the Australian Research Council