Anja Blondeel

Tin doping of silicon for controlling oxygen precipitation and radiation hardness

This paper reviews the impact of doping silicon with substitutional tin impurities on the formation of intrinsic and extrinsic lattice defects. The two major topics covered are (i) the effect on the diffusivity and aggregation/precipitation of interstitial oxygen in Czochralski (CZ) silicon and (ii) the formation of stable radiation defects in irradiated Sn-doped material. As demonstrated, the compressive stress associated with incorporating a large Sn atom on a lattice site is the basic feature governing the interactions with point defects. Consequently, Sn acts as a selective vacancy trap, while, in contrast, not affecting interstitial reactions. This leads to a reduced formation of oxygen thermal donors in n-type Si and lowers the concentration of vacancy-oxygen and divacancy centers in irradiated material. Enhanced oxygen precipitation has been noted around 750°C in p-type CZ silicon. Furthermore, specific Sn-related radiation defects are introduced, which question the use of doping with tin as a technique for substrate hardening.

Deep levels in high-energy proton-irradiated tin-doped n-type Czochralski silicon

A deep level transient spectroscopy study of defects created by 61 MeV proton irradiation of tin-doped n-type Czochralski silicon is reported. A comparison is made with the deep levels observed in irradiated p–n junction diodes fabricated in n-type float-zone silicon, without tin doping. The main conclusions are that in Sn-doped material, at least two additional deep radiation centers are introduced at 0.29±0.01 and 0.61±0.02 eV below the conduction band. From annealing experiments, it is concluded that these electron traps dissociate below 120 °C, which is lower than observed before for Sn–V related levels. It is demonstrated that the introduction rates of the well-known radiation defects are significantly smaller in Sn-doped material.

Optical deep level transient spectroscopy of minority carrier traps in n-type high-purity germanium

Deep levels in n-type high-purity (HP) detector grade germanium are studied using optical deep level transient spectroscopy (ODLTS). In this technique, optical injection (using light of above band gap energy) from the back ohmic contact together with a suitable sample configuration results in the detection of centers in the minority half of the band gap. Six deep minority carrier traps are detected in typical n-type HP germanium which turn out to be the same defects as found earlier in typical p-type HP germanium as majority carrier traps. These deep defects are mainly copper related. A formula is deduced to calculate concentrations from the ODLTS spectra. It is shown that in n- and p-type HP germanium not only the same defects are present but that their concentrations are also comparable.

PHOTOINDUCED CURRENT TRANSIENT SPECTROSCOPY OF DEEP DEFECTS IN N-TYPE ULTRAPURE GERMANIUM

Photoinduced current transient spectroscopy (PICTS) is used to study deep minority carrier traps in n-type ultrapure germanium (shallow concentration of the order 109 cm−3). In this technique, which is a variant of deep level transient spectroscopy (DLTS), a neutral structure with two ohmic contacts applied on two opposite faces of the sample is illuminated through one of the contacts with intrinsic, strongly absorbed light. The current transients which follow after interrupting the photoexcitation are analyzed using classical double lock-in DLTS resulting in the detection of centers in the minority half of the band gap (provided the back ohmic contact is negative). After correcting the PICTS spectra for the temperature dependence of the mobility, six peaks superimposed on a broad background are clearly resolved. The peaks are the same as the ones found earlier in high-purity n-type germanium (shallow concentration of the order 1010 cm−3) using optical DLTS. These peaks are mainly Cu related. A formula to...

Electrical characterization of Ar-ion-bombardment-induced damage in Au/Si and PtSi/Si Schottky barrier contacts

Au/Si and PtSi/Si Schottky contacts were prepared on n-Si(100) substrates which had been previously subjected to an Ar ion bombardment with well defined energies ranging from 100 eV to 1.5 keV. Samples were investigated by current-voltage (I-V) measurements, ballistic electron emission microscopy (BEEM) and deep-level transient spectroscopy (DLTS). Both I-V and BEEM results show that the effective Schottky barrier height (SBH) decreases with increasing Ar ion energy. The lowering of the barrier height is attributed to the bombardment-induced donor-like defects with relatively high densities near the silicon surface. DLTS spectra show the presence of defect levels both in the form of discrete energy levels and as a continuum of states. The oxygen-vacancy pair located at 0.16 eV below the conduction band is the dominant defect for the samples bombarded by 100 and 200 eV Ar ions and its peak signal intensity is similar for the two energies. For 300 eV or higher-energy ion-bombarded samples, other defects develop and become dominant. Their peak signal intensities increase monotonically with Ar ion energy. The variation of the DLTS spectra is in qualitative agreement with the tendency of effective SBH lowering for increasing energy of the bombarding Ar ions.

Lifetime measurements on Ge wafers for Ge/GaAs solar cells — chemical surface passivation

Abstract A rather unknown application of Ge is the use of Ge wafers in Ge/GaAs solar cells which provide the electrical supply of telecommunication satellites. The Ge wafers are used not only as a substrate for the epitaxially grown GaAs-based layers but also as a photovoltaic absorber layer contributing to the photocurrent and thus to the total efficiency of the solar cell. For the latter function the minority carrier lifetime of the Ge wafers is of major importance. Since most minority carrier lifetime measurements on semiconductor wafers have been made on Si (in particular commercial instrumentation and surface passivation) it was necessary to explore this domain for Ge wafers. Two types of lifetime measurements were used in this study: Photoconductive decay measurements (PCD) with contacts on rectangular wafer pieces and contactless microwave detected PCD ( μ -PCD) on complete wafers. In order to measure the bulk recombination lifetime, passivation of the Ge surface is necessary. In this paper the chemical passivation of the Ge wafer surface is studied.

Defect analysis of n-type silicon strained layers.

Abstract The structural and electrical properties of n-type silicon strained layers, sandwiched between Si1−xGex layers, with x=0.15, 0.20 and 0.30 have been investigated using a combination of analytical techniques. Here, the focus is on the application of deep level transient spectroscopy (DLTS) on p–n junction structures, to assess non-radiative generation-recombination centres. It will be demonstrated that successful analysis can only be applied if the edges of the devices are chemically passivated. Finally, it is shown that for low-leakage diodes, the quantum-well properties can, in principle, be extracted from the combined DLTS and capacitance–voltage/capacitance–temperature characteristics.

Quantitative optical variants of deep level transient spectroscopy: application to high purity germanium

Abstract Two optical variants of deep level transient spectroscopy (DLTS) have been quantified and applied to n-type high-purity germanium (shallow concentration 109–1010 cm−3). In both methods, optical injection (using light of above band gap energy) at the back ohmic contact together with a suitable sample configuration (sandwich configuration) results in the detection of centers in the minority half of the bandgap. Different deep hole traps are clearly resolved and identified as mainly Cu related traps with concentrations in the 106–108 cm−3 range. In the first DLTS variant, known as optical DLTS or ODLTS, the spectrum is generated by capacitance transients whereas in the second, it is generated by current transients. The latter method, also known as photo induced (current) transient spectroscopy or PI(C)TS is especially suited for high resistivity or semi-insulating materials which can not be measured with capacitance based DLTS. The formulas to calculate the deep level concentrations from the DLTS spectra are derived and verified experimentally, making these two DLTS variants not only qualitative but also quantitative tools for deep level analysis (comparable to ‘classical’ capacitance DLTS).