Photoconductance Technique Research Articles

Characterization of magnetic Czochralski silicon radiation detectors

Silicon wafers grown by the Magnetic Czochralski (MCZ) method have been processed in form of pad diodes at Instituto de Microelectrònica de Barcelona (IMB-CNM) facilities. The n-type MCZ wafers were manufactured by Okmetic OYJ and they have a nominal resistivity of 1 kΩ cm. Concentrations of oxygen and carbon in MCZ wafers were measured by Secondary Ion Mass Spectroscopy (SIMS) technique to be compared to standard and oxygenated float zone (FZ) silicon material. The diodes were characterized by leakage current and capacitance measurements. The average full depletion voltage, V FD, of the pad detectors is 290±10 V, while the leakage current density at V FD+20 V is 7±3 nA/cm 2. Minority carrier lifetime of MCZ, standard FZ and oxygenated FZ substrates was measured by using a quasi steady-state photoconductance technique. The uniformity of the MCZ wafers was studied before the fabrication by mapping the resistivity of the wafer and after the fabrication process by measuring the full depletion voltage and leakage current as a function of diode position on the wafer. It was found that the fabrication process did not change the resistivity of the material although no donor-killing heat treatment was performed on the wafers.

Effect of Combined Oxygenation and Gettering on Minority Carrier Lifetime in High-Resistivity FZ Silicon

High oxygen concentration in silicon increases the resistance of radiation detectors to high radiation doses. Unfortunately, high-resistivity float zone (FZ) silicon, needed for radiation detectors, has too low an oxygen content. The solution to this is silicon oxygenation. There are different ways of incorporating oxygen into silicon. The most accepted one is by high-temperature diffusion from a thick layer. In this paper, we investigate the impact of this silicon oxygenation technique as well as the application of different gettering techniques to improve the minority carrier lifetime of high-resistivity FZ silicon substrates for radiation detectors. The minority carrier lifetimes before and after surface etching have been measured on samples subjected to different oxygenation and gettering treatments by using a quasi steady-state photoconductance technique. A lifetime improvement efficiency factor is defined for each treatment process. The lifetime efficiency factors behave independently, so that the lifetime efficiency factors associated with different sequential combinations of treatments can be estimated by a multiplicative combination. Different gettering techniques that improve or degrade the minority carrier lifetime are analyzed, and the best options for silicon radiation detector fabrication are determined. Oxygenated silicon with a minority carrier lifetime close to 1 ms can be obtained. © 2004 The Electrochemical Society. All rights reserved.

Improvement of crystalline silicon surface passivation by hydrogen plasma treatment

A completely dry low-temperature process has been developed to passivate 3.3 Ω cm p-type crystalline silicon surface with excellent results. Particularly, we have investigated the use of a hydrogen plasma treatment, just before hydrogenated amorphous silicon carbide (a-SiCx:H) deposition, without breaking the vacuum. We measured effective lifetime, τeff, through a quasi-steady-state photoconductance technique. Experimental results show that hydrogen plasma treatment improves surface passivation compared to classical HF dip. Seff values lower than 19 cm s−1 were achieved using a hydrogen plasma treatment and an a-SiCx:H film deposited at 300 °C.

Damage studies in dry etched textured silicon surfaces

Surface texturing is a more permanent and effective solution to eliminate reflections compared with antireflection coatings in optical devices. In this study texturing was performed using a reactive ion etching technique, reflectance was measured and the resultant damage on the surfaces was monitored through the minority carrier lifetime measurements. High minority carrier lifetime is an indication of low defect centres and is essential for maximum collection efficiency. It is found that the reflectance of the textured cone structures is less than 0.4% at wavelengths from 500 to 1000 nm and shows a minimum of 0.29% at 1000 nm. while the reflectivity from black silicon is around 1% and from hole structures is around 6.8% in the same wavelength range. The quasi-steady-state photo conductance technique was used to measure the effective carrier lifetimes of the textured samples, showing that chemical wet etch damage removal is effective in improving the lifetime of the sample.

Surface passivation of crystalline silicon by Cat-CVD amorphous and nanocrystalline thin silicon films

In this work, we study the electronic surface passivation of crystalline silicon with intrinsic thin silicon films deposited by Catalytic CVD. The contactless method used to determine the effective surface recombination velocity was the quasi-steady-state photoconductance technique. Hydrogenated amorphous and nanocrystalline silicon films were evaluated as passivating layers on n- and p-type float zone silicon wafers. The best results were obtained with amorphous silicon films, which allowed effective surface recombination velocities as low as 60 and 130 cm s −1 on p- and n-type silicon, respectively. To our knowledge, these are the best results ever reported with intrinsic amorphous silicon films deposited by Catalytic CVD. The passivating properties of nanocrystalline silicon films strongly depended on the deposition conditions, especially on the filament temperature. Samples grown at lower filament temperatures (1600 °C) allowed effective surface recombination velocities of 450 and 600 cm s −1 on n- and p-type silicon.

Electronic properties of the metastable defect in boron-doped Czochralski silicon: Unambiguous determination by advanced lifetime spectroscopy

By combining data from temperature- and injection-dependent lifetime spectroscopy (TDLS and IDLS) measured by means of the microwave-detected photoconductance decay technique and the quasi-steady state photoconductance technique, respectively, the exact electronic structure of the metastable defect in standard boron-doped Czochralski (Cz) silicon has been determined. A detailed Shockley–Read–Hall analysis of the entire TDLS curve reveals that the Cz-specific defect acts as an attractive Coulomb center [σn(T)=σn0T−2] which is localized in the upper band-gap half at EC−Et=0.41 eV and has an electron/hole capture cross section ratio k=σn/σp=9.3. The accuracy of this determination manifests itself by the fact that the corresponding IDLS curve can be simulated with the same parameter set.

Surface passivation of n-type crystalline Si by plasma-enhanced-chemical-vapor-deposited amorphous SiCx:H and amorphous SiCxNy:H films

Excellent passivation of n-type crystalline silicon surface is demonstrated by means of intrinsic amorphous silicon carbide (a-SiCx:H) thin films. An optimum CH4/SiH4 ratio is determined, leading to an effective surface recombination velocity, Seff, lower than 54 cm s−1. By adding a constant flow of N2 to the precursor gases, the surface passivation is improved to Seff⩽16 cm s−1. From infrared spectroscopy measurements of these films, it can be deduced that the N2 flow increases the carbon content of the layers for a constant CH4/SiH4 ratio. The dependence of the effective lifetime, τeff, on the excess charge carrier density, Δn, is measured using the quasisteady-state photoconductance technique, and these curves are simulated through an electrical model based on an insulator/semiconductor structure.

Lifetime spectroscopy for defect characterization: Systematic analysis of the possibilities and restrictions

Carrier lifetime is very sensitive to electrically active defects. Apart from detecting the presence of recombination active defects, lifetime measurements allow for a direct identification of defects if the temperature and injection dependence of carrier lifetime is analyzed. In the present work, the Shockley–Read–Hall (SRH) equation is analyzed from a theoretical point of view in order to demonstrate the possibilities and basic restrictions of i_njection- and t_emperature-d_ependent l_ifetime s_pectroscopy (IDLS and TDLS), respectively. To show the variety of attainable curve shapes and to evaluate their suitability for a spectroscopic determination of defect parameters, the general impact of the defect and material parameters on the injection and temperature dependence of SRH lifetime is investigated systematically. From this the basic requirements for an unambiguous defect characterization are derived. The experimental applicability of lifetime spectroscopy is demonstrated by TDLS and IDLS measurements performed on an intentionally molybdenum contaminated Si sample using the microwave-detected photoconductance decay technique and the quasi-steady-state photoconductance technique, respectively. The energy level of 0.335 eV below the conduction band determined from TDLS corresponds well with deep level transient spectroscopy-values reported in literature and could also be determined from the corresponding IDLS curve though not unambiguously.

A contactless photoconductance technique to evaluate the quantum efficiency of solar cell emitters

A new technique, the spectral response of the steady-state photoconductance, is proposed for solar cell characterisation in research and development. The emphasis of this paper is on the evaluation of the carrier collection efficiency of the emitter region based on a simple, two-wavelength approach. We show that in addition to the well-established determination of the wafer recombination properties that results from a long-wavelength photoconductance measurement, detailed emitter quantum-efficiency information can be obtained by performing a second measurement with short-wavelength light. The method is experimentally demonstrated with silicon solar cell precursors having emitters with markedly different levels of surface and bulk recombination losses. The main advantages of the spectral photoconductance technique are that it is fast, contactless, and can be used immediately after junction formation before metallisation; these properties make it very appropriate for routine monitoring of the emitter region, including in-line process control.

Surface passivation of p-type crystalline Si by plasma enhanced chemical vapor deposited amorphous SiCx:H films

Excellent passivation properties of intrinsic amorphous silicon carbide (a-SiCx:H) films deposited by plasma enhanced chemical vapor deposition on single-crystalline silicon (c-Si) wafers have been obtained. The dependence of the effective surface recombination velocity, Seff, on deposition temperature, total pressure and methane (CH4) to silane (SiH4) ratio has been studied for these films using lifetime measurements made with the quasi-steady-state photoconductance technique. The dependence of the effective lifetime, τeff, on the excess carrier density, Δn, has been measured and also simulated through a physical model based on Shockley–Read–Hall statistics and an insulator/semiconductor structure with fixed charges and band bending. A Seff at the a-SiCx:H/c-Si interface lower than 30 cm s−1 was achieved with optimized deposition conditions. This passivation quality was found to be three times better than that of noncarbonated amorphous silicon (a-Si:H) films deposited under equivalent conditions.

Trapping of minority carriers in multicrystalline silicon

Abnormally high effective carrier lifetimes have been observed in multicrystalline silicon wafers using both transient and steady-state photoconductance techniques. A simple model based on the presence of trapping centers explains this phenomenon both qualitatively and quantitatively. By fitting this model to experimental data acquired with a quasi-steady-state photoconductance technique, it is possible to determine the trap density, trap energy, and the ratio between the mean-trapping time and mean-escape time. A correlation between trap density and dislocation density in the material has been found.

Contactless determination of current–voltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data

A simple method for implementing the steady-state photoconductance technique for determining the minority-carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi-steady-state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources. Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi-steady-state photoconductance as a function of incident light intensity, implicit current–voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner.

Effect of rapid thermal annealing on the structural and electrical properties of a silicon-silicon oxide system

An investigation of the electrical and structural properties of rapid thermal annealed Czochralski silicon wafers has been carried out. The electrical properties examined here are the minority carrier lifetime, measured using the laser microwave photoconductance technique, and the donor concentration (Nd), determined by the four-point probe method. Thermal donors were intentionally introduced into the silicon and were found to be completely annihilated by the rapid thermal annealing (RTA) process. The minority carrier lifetime was found to increase significantly for wafers annealed at 900 and 1000 °C. It was concluded that due to the very short annealing time used in this work, a denuded zone was unlikely to form in silicon and be responsible for the increase in the lifetime. Infrared, x-ray photoelectron (XPS) and Auger electron spectroscopies were used for the structural analysis. Auger results showed that higher oxygen concentration could be found in a thicker layer of silicon in annealed wafers, as compared to the as-received, virgin sample. The XPS data showed that the SiO2:Si ratio increased from 0.28 for the virgin sample to 3.5 for wafers annealed at 1000 °C. It is suggested that the Auger and XPS data could be explained by considering oxygen outdiffused from the bulk of the silicon to the native oxide and the silicon next to the native oxide. We believe this is the first investigation of the effect of RTA on the behavior of oxygen and thermal donors, and its influence on the minority carrier lifetime of Czochralski silicon.

Influence of Crystal Thermal History on Surface Recombination Lifetime at Elevated Temperatures in Magnetic-Field-Applied Czochralski Silicon

Recombination lifetime for wafers prepared from two magnetic-field-applied Czochralski (MCZ) silicon crystals with a different body length was analyzed using a laser/microwave photoconductance (LM-PC) technique with an algorithm to separate the surface and bulk components. It has been shown that the surface lifetime dominantly contributes to the dependence of the effective lifetime at elevated temperatures on the crystal length. It is interpreted that the surface lifetime is strongly affected by the grown-in defects which depend on the thermal history, or the crystal length of the silicon crystals.

Effect of ultraviolet irradiation upon the recombination lifetime of silicon wafers covered with a dielectric film

The effect of UV irradiation upon the recombination lifetime of silicon covered with chemical vapor deposition (CVD) oxide has been studied using a laser-microwave photoconductance (LM-PC) technique. It is found that the lifetime changes little after the first UV irradiation, but dramatically decreases following thermal annealing at 500 K for 1 h. Moreover, the lifetime can be cycled up and down by repeated irradiation and thermal annealing. A comparison is made of the UV irradiation effect upon the lifetime of silicon wafers covered with a variety of different dielectric films. It is suggested that UV rechargeable defects are present in a CVD oxide film, like in native oxide and CVD nitride, but are absent in thermal oxide. Finally, it is emphasized that the noncontact LM-PC technique can be a powerful tool to characterize the defects in dielectric films on silicon wafers.

Investigation on Defects in Czochralski Silicon with High-Sensitive Laser/Microwave Photoconductance Technique

High sensitivity of a noncontact laser/microwave photoconductance (LM-PC) technique for defects in heat-treated Czochralski (CZ) silicon crystals was confirmed in comparison with other diagnostic techniques such as Fourier transform infrared spectroscopy (FT-IR) and deep-level transient spectroscopy (DLTS). The energy levels of recombination centers (ET) in samples subjected to a three-step intrinsic gettering (IG) thermal process were obtained with LM-PC although no significant oxygen reduction in the samples was observed with FT-IR. The defect density detection limits of LM-PC and DLTS for samples after the three-step IG process are estimated to be ≤1×107 cm-3 and ∼1×1010 cm-3, respectively. Moreover, the dependence of effective minority-carrier recombination lifetime on defect density was found to be influenced greatly by carbon concentration through the enhancement effect on oxygen precipitation.

Separation of the bulk and surface components of recombination lifetime obtained with a single laser/microwave photoconductance technique

An algorithm for separating the bulk and surface components of recombination lifetime obtained via a contactless single laser excitation/microwave reflection decay measurement is presented. The surface recombination component of lifetime is determined by extrapolating the tail portion of the carrier decay curve to the carrier axis. Although the slope of this curve depends on both surface and bulk properties, it is shown that the y intercept depends only on the surface component of lifetime. A wide range of surface lifetimes, corresponding to surface recombination velocities from 102 to 105 cm/s, and bulk lifetimes from a few microseconds to several hundred microseconds can be measured. An experimental verification of the analysis is presented using microwave absorption/reflection measurements on silicon wafers representing a wide variety of bulk and surface lifetime components.

Transient recovery of minority-carrier lifetime in silicon after ultraviolet irradiation

A recovery process of minority-carrier recombination lifetime after ultraviolet (UV) irradiation has been investigated with a laser-microwave photoconductance technique for silicon wafers with native oxide. It is found that the effective lifetime which greatly increases after UV irradiation recovers to the initial value primarily with an exponential law characterized by a specific time constant called recovery time. The recovery time depends on experimental conditions where, for example, an accumulation effect of UV irradiation is observed. A mechanism of the effective lifetime recovery process is correlated mainly with the behavior of slow states associated with the silicon/native oxide interface.

Noncontact mobility measurements with a laser/microwave photoconductance technique: Temperature dependence

Minority carrier conductance mobility was measured in a noncontact fashion using a laser excitation/microwave reflection photoconductance technique. The mobility was obtained from the surface component of recombination lifetime via measurements of effective lifetime on sample pairs which consist of an oxidized wafer, which is subsequently damaged by lapping, or two lapped wafers with different thicknesses. The experimental temperature dependence of mobility was compared with theoretical predictions given by semiempirical formulas. A good agreement between experimental data and theoretical calculations for both measurement algorithms and n- and p-type conductivity silicon material was obtained.

Deep Level Energy Analysis of Surface and Bulk Defects Using a Noncontact Laser/Microwave Photoconductance Technique

ABSTRACTA noncontact technique for deep level energy analysis has been discussed based on a laser excitation/microwave reflection transient photoconductance procedure. An algorithm for separation of surface and bulk recombination effects was developed to independently determinesurface and bulk energy states. Deep energy levels associated with trapping and recombination processes have been calculated from the temperature dependence of surface recombination velocity and bulk recombination lifetime, based on state occupation statistics. Results have been compared with conventional DLTS data for silicon samples intentionally doped with metals during crystal growth.