Shyam Singh

Behaviour of oxygen in CZ-silicon during 430-630°C heat treatment

Abstract The generation of thermal donors (TDs) depends on initial oxygen concentration, annealing temperature and its duration. An annealing process at 450°C for several hours generates upto ≈1016 cm−3 TDs. We found that maximum concentration of TDs formed at 480°C was ≈1.0×1015 cm−3 which reduces to a minimum of ≈2.6×1014 cm−3 at 510°C suggesting thereby that the TDs got annihilated in this temperature range. The number of TDs now increases with a corresponding increase in annealing time giving birth to new donors (NDs). Activation energy for oxygen diffusion was found to be 0.6±0.1 eV and of silicon self interstitials ≈0.4 eV. Therefore, it is quite logical to conclude that oxygen diffusion at low temperatures depends on the transport of the self interstitials which are chiefly due to the formation of molecule-like oxygen clusters. This confirms the formation and diffusion of molecule-like oxygen clusters in silicon at low temperatures.

Oxygen related donors in Czochralski-grown silicon annealed at 465–650°C

Abstract Generation of new donors (NDs) as a result of annealing of boron-doped CZ-silicon at 650°C under a nitrogen ambient for different durations was studied. The results of the influence of oxygen and carbon on donors generated as a result of annealing treatment to the oxygen rich and carbon lean CZ–Si samples with different resistivities are reported. Behaviour of oxygen related donors from thermal donors (TDs) to NDs in the transition region (465°C–540°C) are also studied. It was found the donor generation rate is very rapid in the early stage of annealing and decreases later on. Carbon and oxygen precipitations as a function of annealing time showed that the donor generation depends on both carbon and oxygen concentration. It was found that carbon enhances the ND formation [1] which is higher in the sample with less carbon concentration. Donors generated as a function of carbon reduction were also studied and a steep rise in the donors generated is attributable to the saturation of carbon reduction.

Grain boundary effect on the electrical properties of boron-doped polysilicon films

The effect of grain boundary width has been accounted for and a modified simple model of average carrier concentration is presented considering the transport mechanism of charge carriers by thermionic emission only. It is found that the electrical properties of polysilicon are very sensitive to doping concentration when the grain size is small and the effect of grain boundary width on electrical properties increases as the grain size decreases. The inclusion of grain boundary width in resistivity and mobility formulae also gives better results near the critical doping concentration. The proposed model gives better agreement between experimental data and theoretical results.

Assessing the Impact of Zero Tilled Wheat Growing in Rice (Oryza Sativa L.)–Wheat (Triticum Aestivum L.) Cropping Systems: The Case of Central Uttar Pradesh in the Indo-Gangetic Plain

Rice–wheat is the most commonly employed cropping system on around 14 million hectares of land extending across the Indo-Gangetic Plain (IGP). The IGP region covers the South Asian countries of Pakistan (2.2 million ha), India (10.5 million ha), Nepal (0.5 million ha) and Bangladesh (0.8 million ha). The major challenge facing the IGPs rice–wheat cropping system is to sustain long-term productivity. This system has a pivotal role in the food security and livelihoods of millions of farmers and workers of populous countries such as India, particularly in central Uttar Pradesh. The systems productivity and economic gains have been consistently decreasing, mainly because of the delayed sowing of wheat after the rice harvest and the fatigued soil condition. The regions farmers lose valuable time for pre-sowing irrigation and field preparation due to the gap of two to three weeks between the harvesting of rice and the planting of wheat. If wheat sowing is delayed beyond the optimal time (by late November), yields plummet at the rate of 30 kg per ha per day. The adoption of resource conservation technologies, such as zero tilled wheat sowing, is considered essential to maintain the productivity of the rice–wheat cropping system. Economic analysis of data for two years from six on-farm demonstrations shows that the zero tillage method of wheat cultivation is the most economical and attractive option for the farming community of central Uttar Pradesh. A high grain yield and reduced cost of cultivation per hectare, reduction in the density of weeds, especially Phalaris minor, and greater water saving were noted in zero tilled wheat sowings compared with conventional practices. As a result of field demonstrations and farmer training programmes, the introduction of zero till drill wheat sowing has expanded rapidly and has made significant contributions to the tillage revolution in the study area.

Formation of oxygen related donors in step-annealed CZ-silicon

The effect of step-annealing necessitated by the difficulties being faced in the long duration annealing treatments to be given to CZ-silicon has been studied. One pre-anneal of 10 h followed by annealing of 10 h causes a decrease in the absorption coefficient for carbon (αc). Oxygen and carbon both accelerate thermal donor (TD) formation process but oxygen plays a dominating role. Three anneals of 10 h each followed by one anneal of 10 h support the view that carbon suppresses the donor formation. The absorption coefficient for carbon decreases after a few number of step-anneals resulting in the transformation of TD to new donor (ND) as brought about by annealing at temperature, > 500°C. It is quite logical to conclude that step-annealing may bring about the same results as obtained on continuous annealing for a longer duration.The results have been fully supported by proper interpretation in the light of existing theories.

Loss of oxygen and carbon during donor formation in CZ-silicon

Due to a lack of proper understanding about the formation mechanism of oxygen related donors during the transition temperature range (465–540°C) which exhibits the transition of TDs to NDs, an attempt has been made to study their behaviour in the present context. We have found the formation and diffusion of molecule like oxygen at low temperature annealing of silicon and observed that second order kinetics of oxygen diffusion holds good. A relatively low value of 0·6 eV has been estimated to be the activation energy for the diffusion of oxygen in silicon which is supposed to be due to the hydrogen passivation.

Formation of oxygen-related donors during transition from thermal donors to new donors in CZ-silicon

CZ-Si has been subjected to different heating schedules in an effort to have a deeper insight about the appearance and disappearance of thermal donors, generation of new donors and thermal acceptors along with the amount of oxygen and carbon precipitated. Annealing of the oxygen rich silicon samples in the temperature range 465 - 540 degrees Celsius for 26 hours demonstrated the three OD stages -- growth region followed by annihilation of TDs and subsequently ND generation. Samples with low oxygen content pre-annealed at 455 degrees Celsius for 24 hrs and annealed at 470 degrees Celsius for varying annealing duration showed the complete absence of the first stage of rapid growth of TDs, but the pre-annealed samples with both low and high oxygen contents and subsequently annealed in the temperature range 430 - 630 degrees Celsius for short duration exhibited the dominance of thermal acceptors beyond the annealing temperature corresponding to maximum ND generation. The pattern of oxygen and carbon precipitation for such samples is also different. Activation energy for oxygen diffusion and self interstitials in silicon come out to be 0.6 eV and 0.4 eV respectively and the diffusion coefficient of oxygen in the pre-annealed samples and annealed at 470 degrees Celsius for different durations to be 6.18 by 10-19 cm2s-1. Oxygen reduction is governed by second order kinetics.

TD formation in CZ-silicon annealed at 450 degrees C in air ambient

Exact kinetics of donor formation and possible number of oxygen atoms in a single TD are two aspects out of many problems related with oxygen related donors in CZ-silicon, which carry a mark of interrogation. Boron doped p-type silicon wafers were annealed in air ambient at 450 degrees C for different durations and subjected to resistivity and FTIR studies. It was observed that ambients do not affect the process of TD generation. Successive increase in annealing times results in the exponential growth of donors with a maximum of approximately 1.79 X 1017 cm-3 obtained in our samples annealed for 55 hours only. Annealing also caused a gradual decrease in absorption coefficient. Maximum observed value of oxygen and carbon precipitations was 2.362 ppma and 1.100 ppma respectively. The diffusion coefficient for oxygen was found to be approximately 4.17 X 10-19 cm-9S-1. The oxygen and carbon reduction followed the second order kinetics. The activiation energy was approximately 0.823 eV and the number of oxygen atoms in a single TD may be 7.

Some aspects of CdTe

This article mainly deals with the exact origin of 0.15 - 0.20 eV defect level in CdTe--a controversial issue under debate for more than three decades--and some investigations of deep and interface levels in polycrystalline n-CdTe. The first issue still stands unresolved amidst conflicting views from various workers, thereby leaving it surrounded by an air of uncertainty. The results of a study of deep and interface states in Mo-polycrystalline n-Schottky barriers using a deep level spectrometer DLS-82E (Hungary), both in differential DLTS and level profile mode, are also presented. Three deep levels with activation energies of 0.2, 0.43 and 0.65 eV and capture cross-section of the order of 10-18 cm2 detected in the bulk of layers are discussed.

Kinetics of thermal donors and new oxygen donors in silicon

A model for the oxygen-related donors in Cz-silicon with emphasis on their generation, reduction, and subsequent behavior in the transition region has been developed. A modified SiOx model is presented to account for the new oxygen donors. A schematic diagram of the formation of donors and related defects so developed in silicon from the un-annealed to the annealed stage is also given.