DLTS and FTIR study of quenching induced defects in germanium

Due to the high carrier mobility in Ge, it is more and more used as active semiconducting layer in advanced electronic devices on Si substrates [1]. Successful growth, doping and further processing of Ge requires however a good understanding of the intrinsic point defect properties that are unfortunately not well known. The present paper reports on the progress of an effort to determine the formation energy and diffusivity of the vacancy in Ge using thermal quenching techniques [2]. Experimental data on the thermal equilibrium concentration and diffusivity of vacancies in Ge are scarce and most are more than 40 years old. Most of the experimental data were obtained based on thermal quenching experiments assuming that the formed acceptors are due to quenched-in vacancies so that their concentration and formation energy can be determined from measured resistivity changes. The formation energy of the vacancy in its different charge states has recently also been calculated using ab initio calculations which showed that the (double) negatively charged vacancy has the lowest formation energy of about 2 eV in good agreement with the acceptor formation energy determined from the quenching experiments. Based on vacancy mediated dopant diffusion studies, Brotzmann et al [3] also concluded that the double negatively charged vacancy is the most probable charge state of the vacancy. In this contribution, the quenched-in acceptors are studied using deep-level transient spectroscopy. As Cu is known as contaminant which is difficult to avoid when quenching Ge, the electric properties of the quenched-in acceptors are carefully compared with those of substitutional Cu. Although at first glance similarities are striking, remarkable differences are also observed and discussed. [1] J. Vanhellemont and E. Simoen, J. Electrochem. Soc. 154 (2007), p. H572. [2] J. Vanhellemont, J. Lauwaert, A. Witecka, P. Spiewak, I. Romandic and P. Clauws, Physica B 404 (2009), p. 4529. [3] S. Brotzmann and H. Bracht, J. Appl. Phys. 103 (2008), p. 033508

Temperature-independent slow carrier emission from deep-level defects in p-type germanium

In the deep-level transient spectroscopy (DLTS) spectra of the 3d-transition metals cobalt and chromium in p-type germanium, evidence is obtained that hole emission from defect levels can occur by two parallel paths. Besides classical thermal emission, we observed a second, slower and temperature-independent emission. We show that this extra emission component allows determining unambiguously whether or not multiple DLTS peaks arise from the same defect. Despite similar characteristics, we demonstrate that the origin of the non-thermal emission is not tunnelling but photoionization related to black-body radiation from an insufficiently shielded part of the cryostat

Field-enhanced electron capture by iron impurities in germanium

The dependence of electronic properties of a deep level on electric field is relevant for better understanding of its impact on device characteristics. However, direct observation of the field assisted free carrier capture in DLTS is difficult, since in filling pulse experiments, during the applied pulse, no electric field is expected in the neutral part of the semiconducting substrate. For this reason isothermal DLTS measured as a function of pulse duration is an accurate technique to determine capture cross-sections in absence of electric field. On the other hand, when observing the emission of a carrier in conventional DLTS, an electric field ís present. In this work we applied the double pulse DLTS technique to measure the emission rates of Fe(2-/-) for different temperatures and electric fields. Data analysis revealed that an electric field affects the emission rate mainly through the pre-exponential factor, which is proportional to the capture cross section. An empirical electrical field dependence of the electron capture cross section for a negatively charged iron impurity was deduced

Assessment of model based (input) impedance, pulse wave velocity, and wave reflection in the Asklepios Cohort

Objectives : Arterial stiffness and wave reflection parameters assessed from both invasive and non-invasive pressure and flow readings are used as surrogates for ventricular and vascular load. They have been reported to predict adverse cardiovascular events, but clinical assessment is laborious and may limit widespread use. This study aims to investigate measures of arterial stiffness and central hemodynamics provided by arterial tonometry alone and in combination with aortic root flows derived by echocardiography against surrogates derived by a mathematical pressure and flow model in a healthy middle-aged cohort. Methods : Measurements of carotid artery tonometry and echocardiography were performed on 2226 ASKLEPIOS study participants and parameters of systemic hemodynamics, arterial stiffness and wave reflection based on pressure and flow were measured. In a second step, the analysis was repeated but echocardiography derived flows were substituted by flows provided by a novel mathematical model. This was followed by a quantitative method comparison. Results : All investigated parameters showed a significant association between the methods. Overall agreement was acceptable for all parameters (mean differences: -0.0102 (0.033 SD) mmHg*s/ml for characteristic impedance, 0.36 (4.21 SD) mmHg for forward pressure amplitude, 2.26 (3.51 SD) mmHg for backward pressure amplitude and 0.717 (1.25 SD) m/s for pulse wave velocity). Conclusion : The results indicate that the use of model-based surrogates in a healthy middle aged cohort is feasible and deserves further attention

Direct estimation of capture cross sections in the presence of slow capture: application to the identification of quenched-in deeplevel defects in Ge

Quenching experiments have been performed on both n- and p-type Ge in a dedicated furnace using infrared lamp heating. The capture and emission characteristics of the induced deep-level defects in the quenched samples were investigated by means of deep level transient spectroscopy. For all defect levels, a high impact of capture in the transition region (slow capture) was found. An empirical approach to analyse this effect is presented, which allows to extract reliable capture cross section parameters. The defect parameters thus obtained were compared with previously published data and it was found that some prominent quench-inginduced deep levels are related to metal impurities (Cu and Ni), while others may be vacancy-related

A temperature independent emission component in the capacitance transients of deep-level defects in germanium

The last decades germanium is regaining importance as active layer in semiconductor technology because of the high (drift) mobility of carriers. Most of the interesting properties of this material stem from impurities and point defects. Some of these impurities (f.e. transition metals) give rise to deep levels in the band gap, which can reduce the carrier lifetime and therefor need to be studied. Due to its high sensitivity, deep level transient spectroscopy (DLTS) is an excellent technique to detect and identify these impurities and defects. In this work, p-type germanium wafers have been quenched or intentionally contaminated with transition metals and studied by DLTS. At low temperatures, a quasi-constant emission component has been observed at high time windows (oder of seconds). This component can be associated with a tunneling emission of holes from a defect state towards the valence band. By considering a single quantum well and using the model proposed by Letartre et al. [1], experimental evidence of this tunneling effect in germanium will be given and the electrical properties of these defects will be examined. [1] X. Letartre, D. Stiévenard, M. Lannoo and E. Barbier, J. Appl. Phys. 69, 7336 (1991

Electronic properties of manganese impurities in germanium

The electronic properties of manganese in crystalline germanium have been investigated by means of deep level transient spectroscopy (DLTS). Mn was diffused in the material by a thermal treatment at 700 degrees C. Next to the deep levels of nickel and copper, which are known contaminants in Ge treated at high temperature, three not previously reported levels were observed. These two hole and one electron traps, with apparent energy level positions at EV +0.136 eV, EV +0.342 eV and EC - 0.363 eV, were assigned to substitutional Mn. The analysis of the carrier capture cross-sections, the absence of field-assisted emission and the observation of the Mn2- electron paramagnetic resonance spectrum in n-type Ge: Mn at low temperature are all compatible with Mn introducing two acceptor and one donor levels in the band gap of Ge