Thermal Donor Formation Research Articles

Interstitial oxygen behavior for thermal double donor formation in germanium: Infrared absorption studies

The kinetics of the reduction of interstitial oxygen (Oi) due to the formation of thermal double donors (TDDs) upon heat treatment in an oxygen-rich Ge crystal were investigated at various temperatures. Specimens were prepared from a Ge crystal with oxygen at a concentration of 4–5 × 1017 cm−3 grown by a new Czochralski method and were heat-treated in the temperature range 300–500 °C. Shrinkage of a dissolved oxygen absorption peak at 855 cm−1 and simultaneous development of a thermal double donor peak at 780 cm−1 were observed by infrared absorption spectroscopy at room temperature. The formation of TDDs was also detected electrically. Reduction of dissolved oxygen concentration upon the heat treatments was kinetically analyzed. The activation energy of the reduction of Oi concentration was evaluated to be 1.7 and 2.0 eV in the early and prolonged stages of the heat treatment, respectively, close to that of Oi diffusion. From the results, TDD development process was indicated to be the merge reaction of Oi-Oi to form oxygen dimers (Odimer) in the early stage of the heat treatment and further enlargement of Odimer to TDDs by absorbing Oi in the prolonged stage of the heat treatment in accordance with the On-2NN model.

Silicon homojunction solar cells via a hydrogen plasma etching process

We report on the one-step formation of an efficient Si homojunction solar cell produced by a simple exposure of p-type Si wafers to low-temperature inductively coupled hydrogen plasma. The formation of oxygen thermal donors during hydrogen plasma treatment is responsible for the conductivity type conversion and the final formation of Si homojunction. The hydrogen plasma etching with suppressed heavy ion bombardment results in a relatively flat surface, which is favourable for deposition of passivation layers such as silicon nitride. The integrated Si homojunction solar cell consisting of Al/p-c-Si/n-c-Si/SiN/Al-grid has demonstrated a maximum photovoltaic conversion efficiency of 13.6%.

Windmill-like Structure in Cz-Si

Windmill-like structures in as-grown Cz-Si wafers were characterised by several electrical and optical methods, such as 4PP resistivity, FTIR, SIRD and microwave PCD techniques. All these methods revealed the similar shape and size of the observed structure. Additionally, those windmill-like structures could be reproduced in the temperature distribution at the solidification interface numerically simulated in a certain time window using parameters being typical for the Czochralski process. The dependence of the minority carrier lifetime on resistivity in the investigated slices can be well fitted in the frame of the extended SHR theory using trap energies of EV,C ± (200-300) meV and a large asymmetry in the capture cross sections. Such defects may be oxygen-related double thermal donors with trap energies higher than EC – 200 meV. The FTIR map approves an enhanced concentration of Oi needed for an enhanced thermal donor formation in the windmill cogs and is well-correlated to the shear stress distribution obtained by SIRD Thus, we conclude that a windmill-like structure is formed mainly due to a non-ring-like distribution of oxygen at the solid-liquid interface.

Diode Characteristics and Thermal Donor Formation in Germanium-Doped Silicon Substrates

not Available.

Dimers as Fast Diffusing Species for the Aggregation of Oxygen in Boron-Doped Czochralski Silicon: Formation of New Thermal Donors

The thermal behaviors of oxygen-related complexes in boron doped Czochralski Silicon (Cz-Si) wafers at 450°C and 800°C were investigated using Fourier transform infrared spectroscopy (FTIR) and Hall mobility measurements. Activation of thermal donors (TDs) at 450°C leads to a decrease of both mobility and majority carrier concentration using the four point probes configuration of Van Der Pauw. It was found that annealing at 450°C would possibly affect the electronic properties of the Si wafers via the formation of interstitial dioxygen defects (IO2i), which exhibit an IR absorption band positioned at 545 cm–1. A strengthening of the IR bands peaking at around 1595 cm–1, 1667 cm–1, 1720 cm–1 and 1765 cm–1 occurs at 450°C, while they disappear at 800°C. At high temperatures, the precipitation of interstitial oxygen becomes predominant over all other oxygen-related reactions. The dynamic of oxygen-thermal donor generation-annihilation in Cz-Si involving the formation of small oxygen clusters is discussed.

Oxygen in Ge crystals grown by the B2O3 encapsulated Czochralski method

Local vibrations of oxygen in Ge crystals grown by the Czochralski method adopting liquid-B2O3 encapsulation and GeO2 powder doping were investigated by Fourier-transform infrared spectroscopy. Strong absorption peaks at 855cm−1, originating in local vibration of interstitially dissolved oxygen Oi as Ge–Oi–Ge quasi-molecules, developed depending on the doped amount of GeO2. Similarly, an absorption peak related to the combined vibration of Ge–Oi–Ge was found at 1264cm−1 and the conversion factor from the peak intensity to the oxygen concentration was evaluated to be 1.15×1019cm−2. By prolonged annealing at 350°C an absorption peak developed at 780cm−1, indicating formation of oxygen-related thermal donors. From the variations of carrier density and oxygen concentration, one donor was found to possess about 15 Oi atoms.

Coefficient of Diffusion in Crystals of Si<sub>1-x</sub>Ge<sub>x</sub>: Role of Preexponential Factor

In the present work the results of theoretical analysis of the process of diffusion of covalently bonded atoms (interstitial oxygen atoms) in Si1-xGex alloys are presented. The diffusion coefficient (activation energy and pre-exponential factor) was calculated by means of quantum-chemical simulations (Hartree–Fock, NDDO, PM5) and the dependences of the activation energy and pre-exponential factor on Ge atoms concentration (x) were analyzed with the use of the percolation theory. The study has revealed that the diffusivity of impurities (defects) in alloys can decrease considerably at low concentration (x<0.05) of a minor alloy component and this variation results from the fact that the pre-exponential factor depends on the concentration of component elements of the alloy. The alloy-induced decrease in the pre-exponential factor is associated with removal of the degeneracy of the number of equivalent diffusion paths. It is found that a sharp decrease in the pre-exponential factor causes experimentally observed decreases in the coefficient of diffusion of interstitial oxygen atoms and in the rate of formation of oxygen thermal donors in Si1‑xGex crystals at x~0.01.

Carrier Lifetime Studies in Diode Structures on Si Substrates with and without Ge Doping

Results are presented of a comparative study of diodes processed on n-type Cz grown Si substrates without and with Ge doping concentration of about 1019 cm-3 and 1020 cm-3. In order to investigate thermal donor formation, isothermal annealing at 450°C for 0.5 – 5 h was carried out. As processed diodes were also irradiated with 2 MeV electrons with fluences in the range between 1014 and 1017 e/cm2 to investigate the Ge doping influence on irradiation induced defect formation. Diodes after thermal and radiation treatments have been investigated by combining different techniques.

Study of oxygen clustering in Czochralski silicon at 450 °C–800 °C: correlation with thermal donors formation

AbstractThe understanding of the generation‐annihilation phenomena of oxygen‐related thermal donors (TDs) in p‐type Czochralski silicon is of great importance for device fabrication and operation in microelectronic and photovoltaic industry. In this work, we report on oxygen clustering‐based activation and partial annihilation of TDs in Cz‐Si submitted to thermal annealing under nitrogen at 450 °C‐800 °C for different thermal plateau exceeding 4 hours. The effect of thermal annealing on interstitial oxygen and dimer (Oi and O2i) vibration modes appearing in the 580 cm‐1‐500 cm‐1 spectral range is studied. We found that heat treatment at 450°C and 650 °C give rise to new thermal donors (NDs) that are located at 540 cm‐1 and 532 cm‐1, respectively. Subsequent to heat treatment above 650 °C, we depict the formation of new thermal oxygen defects having an IR signature in the 1800 cm‐1‐1500 cm‐1 spectral range, and which are especially sensitive to the thermal treatment duration. Resistivity measurements confirm that several of them are electrically active. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Fast and Easily Implemented Method for Interstitial Oxygen Concentration Mapping Through the Activation of Thermal Donors in Silicon.

Abstract In this paper a method is presented to accurately and readily measure the interstitial oxygen concentration in silicon. This method relies on the modification of the Si resistivity after the generation of some oxygen-based Thermal Donors. The method is made very accurate due to the strong dependence of the thermal donors formation rate on the interstitial oxygen concentration. The presented procedure is non destructive and only requires a resistivity measurement setup and a standard 450 °C air furnace. Very high spatial resolution mappings can be achieved using up-to-date resistivity measurement tools.

On conductivity type conversion of p-type silicon exposed to a low-frequency inductively coupled plasma of Ar + H2

The treatment of an Ar + H2 plasma generated by a low-frequency inductively coupled plasma system at 500 °C introduces an n-type region (of average electron concentration ∼1015 cm−3) on a Czochralski p-type substrate, forming a deep p–n junction. Examination by an electron microscope shows that the plasma treatment produces uniform nanocones on the surface and some defects, such as dislocations and platelets, in the subsurface. All these observed results are hydrogen-related. The conductivity type conversion is due to the formation of hydrogen-enhanced oxygen-related thermal donors (OTDs) as well as hydrogen-incorporated shallow thermal donors. The OTD-related signals are directly observed in the infrared absorption spectra. Both donors are annihilated after annealing at 550 °C for 10 min, resulting in conductivity recovery from n-type to original p-type. The electrical properties of the as-formed junction are investigated using current versus voltage (I–V), capacitance versus voltage (C–V) and Hall effect measurements. On this basis, the junction depth, carrier profile and hydrogen diffusion behaviour are studied. Moreover, a clear photovoltaic effect of the junction has been observed through the Suns-Voc and illuminated I–V tests.

Germanium doping for improved silicon substrates and devices

During the last decade, the 300 mm silicon wafer has been optimized and one is studying the move to 450 mm crystals and wafers. The ever increasing silicon crystal diameter leads to two important trends with respect to substrate characteristics: the interstitial oxygen concentration decreases while the size of grown in voids and crystal originated particles (COPs) in vacancy-rich crystals is increasing. The first effect is due to the large melt in which movements have to be controlled and partly suppressed by the use of magnetic fields. This magnetic confinement leads to a more uniform dopant incorporation but at the same time to a more limited transport of oxygen from the quartz crucible to the melt and the growing crystal. The reduced interstitial oxygen concentration and the lower thermal budget of modern device processing leads to strongly reduced oxygen precipitation and thus internal gettering capacity. The increasing COP size (accompanied by a decreasing density) is caused by the decreasing pulling rate and thermal gradient that have to be used in order to avoid dislocation formation. The slower cooling of the crystal leads to a decreased void nucleation rate and at the same time to an increased thermal budget for void growth as well as a larger number of vacancies available per void. In the present paper the effect of germanium doping in the range between 10 16 and 10 19 cm −3 on COP formation and oxygen precipitation is discussed and illustrated. Also the beneficial effect of germanium doping with respect to wafer breakage during processing, with respect to the suppression of thermal donor formation and with respect to improving device radiation hardness is addressed.

Electrical properties of thermal donors formed in silicon under elastic tensile stress

The electrical properties of thermal donors formed in the bulk and near-surface regions in silicon samples with (3–9) × 1017 cm−3 oxygen concentrations under elastic tensile stress σ of about 1 GPa have been studied. The original method allowing us to control an introduced elastic tensile stress during the thermal donor’s formation at T = 450°C by a double-crystal X-ray diffractometer has been used. The formation of thermal donors in silicon with a high oxygen concentration of 9.3 × 1017 cm−3 under tensile stress has been found to be less effective than in silicon with a low oxygen concentration of (3–5) × 1017 cm−3. Single-charged donors are formed in silicon with a low oxygen concentration under tensile stress while double-charged donors are formed in silicon with a high oxygen concentration.

What Do We Know about Hydrogen-Induced Thermal Donors in Silicon?

The hydrogen-plasma-accelerated formation of shallow thermal donors in silicon has been studied for a wide range of doping concentration and interstitial oxygen content by electrical and spectroscopic techniques. The plasma-hydrogenated material has been heat treated for different times in the temperature range of . It is shown that, besides oxygen thermal donors (OTDs), hydrogen-related shallow thermal donors (STDHs) also play a crucial role in the hydrogen-assisted creation of excess carriers. The impact of different factors on the introduction rate of the shallow donors will be discussed, whereby a strong role is played by the doping concentration and type (i.e., the Fermi-level position during the thermal anneal in air). Generally, shallow donor formation is faster in p- compared to n-type Si, which is associated with the different charge state of H. From combined deep-level transient spectroscopy and Fourier transform infrared absorption spectroscopy, it is concluded that the additional free carriers are contributed by both STDH and OTD centers, so that H not only plays a catalytic role but actively takes part in the donor formation, depending on the experimental conditions. Finally, from our data some conclusions can be made regarding the nature of the STDHs, which is still a matter of debate.

Radiation Hard Silicon for Medical, Space and High Energy Physics Applications

The objective of this paper is to give an overview on how silicon particle detector would survive operational in extremely harsh radiation environment after luminosity upgrade of the CERN LHC (Large Hadron Collider). The Super-LHC would result in an integrated fluence 1×1016 p/cm2 and that is well beyond the radiation tolerance of even the most advanced semiconductor detectors fabricated by commonly adopted technologies. The Czochralski silicon (Cz-Si) has intrinsically high oxygen concentration. Therefore Cz-Si is considered as a promising material for the tracking systems in future very high luminosity colliders. The fabrication process issues of Cz-Si are discussed and the formation of thermal donors is especially emphasized. N+/p-/p+ and p+/n-/n+ detectors have been processed on magnetic Czochralski (MCz-Si) wafers. We show measurement data of AC-coupled strip detectors and single pad detectors as well as experimental results of intentional TD doping. Data of spatial homogeneity of electrical properties, full depletion voltage and leakage current, is shown and n and p-type devices are compared. Our results show that it is possible to manufacture high quality n+/p-/p+ and p+/n-/n+ particle detectors from high resistivity Czochralski silicon.

Thermal donor formation in CZ-silicon with reference to dimers, trimers and V–O interaction

Abstract Silicon wafers with different initial oxygen and carbon concentrations annealed in air ambient at 650 °C revealed that the new donor concentration is more for the sample with high carbon content at same values of oxygen reduction. Vacancy–oxygen interaction plays an important role resulting in the interaction of dimers and trimers with the vacancies consequently forming V m –O n defects. These defects get trapped at silicon interstitial releasing a dimer and trimer. Dimers also interact with other defects such as C s O defects in CZ-Silicon. Thermal donors (TDs) are formed through the aggregation of rapidly diffusing dimers. Dimers also encounter with third O i atom and form trimers. They may encounter with next O i and 4O i atoms experience sufficient lattice strain to switch them into di-y-lid structure. A new model based on trimers is proposed to account for the formation of thermal donors. This article aims at minimizing the uncertainty providing amicable explanation to the proposed structural model.

Influence of radiation defects on formation of thermal donors in silicon irradiated with high-energy helium ions

Isothermal annealing at temperatures from 370 to 410 °C was used for investigation of radiation enhanced formation of thermal donors in oxygen-rich FZ silicon irradiated with 7 MeV helium ions. Results show that formation of these defects is caused by hydrogen atoms which are diffusing from the anode contact. Hydrogen enhances diffusivity of interstitial oxygen and allows it to react with radiation defects containing vacancies and oxygen to form a new type of thermal donors. Their distribution is then given by concentration profile of radiation damage and diffusion length of hydrogen atoms. This effect occurs only in oxygen-rich silicon annealed in air and is absent in oxygen–lean silicon and samples annealed in vacuum.

What do We Know About Hydrogen-Induced Thermal Donors?

The hydrogen-plasma-accelerated formation of shallow thermal donors in silicon has been studied for a wide range of doping density and interstitial oxygen content [Oi] by electrical and spectroscopic techniques. The plasma-hydrogenated material has been heat treated for different times in the temperature range of 275 to 500 oC. It is shown that besides Oxygen Thermal Donors (OTDs) also hydrogen related Shallow Thermal Donors (STDHs) play a crucial role in the hydrogen-assisted creation of excess carriers. The impact of different factors on the introduction rate of the shallow donors will be discussed, whereby a strong role is played by the doping density and type, i.e., the Fermi level position during the thermal anneal in air. Generally, shallow donor formation is faster in p- compared with n-type Si, which is associated with the different charge state of H. From combined Deep Level Transient Spectroscopy (DLTS) and Fourier Transform Infrared (FTIR) absorption spectroscopy it is concluded that the additional free carriers are contributed by both STDH and OTD centers, so that H not only plays a catalytic role but actively takes part in the donor formation, depending on the experimental conditions. Finally, from our data some conclusions can be made regarding the nature of the STDHs, which is still a matter of debate.

Evidence of energy levels due to nitrogen dimers in silicon

Nitrogen-and-boron doped Czochralski Si samples are converted into n-type by annealing at 600 °C due to formation of Shallow Thermal Donors (STDs). The temperature dependence of the electron concentration, measured by Hall effect down to the liquid Helium temperature, is not however consistent with existence of only STDs and compensating boron acceptors; there is clearly an extra acceptor species with a level close to the conduction band, and of a concentration comparable to the total concentration of nitrogen. This acceptor centre is thus identified with the major nitrogen species, N 2 and N 2O. These two species can be distinguished by using samples from the seed-end and tail-end of a crystal: the level due to N 2 is about E c −27 meV, and that due to N 2O about E c −33 meV.

Enhanced Formation of Thermal Donors in Germanium Doped Czochralski Silicon Pretreated by Rapid Thermal Annealing

The thermal donor formation at 425oC - 450oC in Ge doped Czochralski (GCZ) silicon having about 1016 cm-3 Ge content pretreated by rapid thermal annealing (RTA) and conventional furnace annealing (CFA) has been investigated using low-temperature infrared spectroscopy (LT-IR). The measurements prove that lightly Ge doping can enhance the formation of thermal double donors in the initial stage of the low temperature annealing after RTA process. Ge induced additional grown-in oxygen precipitates during silicon ingot growth and the abundant self-interstitials during RTA may be the reason for the enhancement. However, after extending the annealing time at the low temperatures, the thermal donor concentration in the GCZ silicon is lower than that in the conventional CZ silicon. In final, the mechanism is also discussed.