E. Gaubas

Comparative Study of Carrier Lifetime Dependence on Dopant Concentration in Silicon and Germanium

The increasing use of thin germanium layers on silicon substrates as active layers in advanced electronic devices is the motivation for a comparative study of the dependence of the carrier recombination characteristics on excitation and dopant concentration in both semiconductors. Bulk and surface components of the carrier lifetime are separated by studying samples with different geometries and surface finishing. The bulk carrier lifetime is determined as a function of dopant concentration and in the case of germanium also as a function of the excitation level. The bulk lifetime observations are simulated by combining the lifetime according to Shockley-Read-Hall (SRH) theory with the lifetime resulting from Auger recombination above a doping concentration threshold. It is shown that the bulk carrier lifetime behavior in Si and Ge is very similar. Carrier lifetime changes in Si can be well described by a combined SRH and Auger recombination model, while lifetime variations in Ge fit well the SRH model, taking into account the lifetime dependence on the injection level. It is shown that the lifetime dependence on dopant concentration in germanium can be phenomenologically described by assuming a linear increase of recombination center concentration with dopant concentration. The results also illustrate the importance of surface passivation and of material quality which depends on the fabrication technology, when comparing carrier lifetimes obtained with different equipment and on different samples.

Dependence of carrier lifetime in germanium on resisitivity and carrier injection level

The dependence of carrier lifetime on resistivity and carrier injection level in germanium crystals is studied using microwave probing of optically excited samples. Bulk lifetimes in the range between 30ns and 500μs are measured. The carrier lifetime decreases with decreasing resistivity and increases with increasing excitation level. Possible mechanisms are discussed and it is shown that the carrier recombination transients are driven by a system of recombination and trapping centers.

Semi-insulating GaN and its evaluation for α particle detection

Abstract The application of semi-insulating GaN for detection of ionising particles, specifically α-particles, is presented. The electrical properties of GaN epitaxial layers have been investigated and the space charge limited and Ohmic currents were observed. A microwave method was used for temporal measurements of the photocurrent. Transient behaviour of the injection current and photo-response was observed, with a wide range of instantaneous time constants. The charge collection efficiency was measured to be approximately 100% and the role of inhomogeneities in the charge collection peculiarities was analysed.

Excess carrier cross-sectional profiling technique for determination of the surface recombination velocity

Abstract This paper presents a modified excess carrier profiling technique using carrier decay cross-sectional scans of wafers using a narrow excitation fiber guided beam and by using infrared light as well as microwave probes. The theoretical principles of the technique are based on the analysis of the depth variation of decay shape and the amplitude.

Investigation of recombination parameters in silicon structures by infrared and microwave transient absorption techniques

Contactless techniques of infrared and microwave absorption by free carriers for the monitoring of silicon structures are described. Theoretical principles of photoconductivity decay analysis and methodology for the determination of recombination parameters are given for both homogeneous and non-homogeneous excess carrier generation. Different approximations (the methods of decay amplitude - asymptotic lifetime analysis, the simulation of the whole decay curve, the variation of effective lifetime with wafer thickness and the asymptotic lifetime measurement for stepwise varying parameters in layered structure) corresponding to real experimental conditions for various structures and treatments of materials, which are important for microelectronics, are discussed. The determined recombination parameters in the range of bulk lifetime , velocity of surface recombination and diffusion coefficient are illustrated for Si wafers obtained by various doping and preparation processes. The necessity to consider carrier trapping effects and nonlinear recombination processes is demonstrated by the analysis of experimental results obtained at different excitation levels for carrier concentrations in the range . The possibility of extracting the parameters of the traps (with activation energy values , and ) from the temperature-dependent asymptotic carrier lifetime measurements is illustrated for neutron transmutation doped wafers.

Mapping of GaAs and Si wafers and ion-implanted layers by light-induced scattering and absorption of IR light

The methods of light-induced absorption and diffraction have been used to investigate the uniformity of Si and GaAs wafers and ion-implanted Si. Metastable annealing of defects, lifetime modification by swirl and radiation defects and anomalous depth penetration of ion-implantation-created defects have been demonstrated in Si. The mapping of GaAs wafers has been performed with picosecond and nanosecond light pulses and light absorption mechanisms are discussed.

Carrier and defect dynamics in photoexcited semi-insulating epitaxial GaN layers

Transients of fast free-carrier recombination and of multitrapping processes, determined by different types of defects, have been traced by photoluminescence (PL) and contact photoconductivity (CPC) in semi-insulating GaN epitaxial layers. To eliminate effects caused by the electrodes, the CPC decays were supplemented with noninvasive microwave absorption transients. The lifetimes of fast recombination and initial free-carrier capture processes were evaluated using ultraviolet (UV) time-resolved photoluminescence transients. The UV PL band peaked at 3.42 eV with contributions from both stimulated and spontaneous emission was attributed to band-to-band recombination. At the highest excitations, the initial PL decay time exhibited a value of 880 ps due to nonradiative free-carrier recombination. The radiative centers were revealed in continuous-wave PL spectra, where the UV band was accompanied with the bands of blue (B) PL, peaked in the range of 2.82–3.10 eV, and yellow (Y) PL, peaked at 2.19 eV, ascribed...

DETERMINATION OF RECOMBINATION PARAMETERS IN SILICON WAFERS BY TRANSIENT MICROWAVE ABSORPTION

Principles of measurement of photoconductance transients by time‐resolved microwave absorption and reflection mode are presented. The microwave transmission (absorption) mode is a new implementation of the time‐resolved microwave conductivity method. This instrument is more sensitive with respect to microwave response signal and less critical to instabilities induced by phase modulation of the response. An adjustment of the measurement system into a local resonance for each particular sample under investigation and the whole set of experimental conditions is crucial to ensure the highest sensitivity and reliability of the instruments. The waveguide slot resonance antenna provides mapping of recombination parameters in silicon wafers of thickness d≥50 μm and resistivity ρ≥1 Ω cm with a spatial resolution of 1–2 mm. Theoretical models and validity of the approximations for carrier decay analysis and determination of the recombination parameters are discussed. The nonlinearities of the recombination processe...

Radiation-defect-dependent photoconductivity transients and photoluminescence in semi-insulating GaN

Effect of radiation defects on the photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN∕sapphire substrates. Manifestation of defects induced by 10keV x-ray irradiation with the dose of 600Mrad and 100keV neutrons with the fluence of 5×1014cm−2 have been revealed through steady-state and pulsed PL as well as through contact photoconductivity and microwave absorption transients. Synchronous decrease of the PL intensity of yellow, blue, and ultraviolet bands peaked at 2.19, 2.85, and 3.42eV, respectively, with density of radiation-induced defects is observed. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess-carrier multi-trapping. The decay fits the stretched exponent approximation exp[−(t∕τ)α] with the different factors α in as-grown material (α≈0.7) and irradiated samples (α≈0.3).

Recombination activity of iron‐related complexes in silicon studied by temperature dependent carrier lifetime measurements

Carrier recombination centers related with iron complexes in p‐type silicon are studied by microwave and light‐induced absorption techniques. Both thermal‐ and photoactivation are used to decompose iron–boron pairs and to study the impact on the recombination lifetime. Due to photodissociation of iron–boron pairs the lifetime increases for high level injection. Efficient recombination occurs via an acceptor level at Ec−0.29 eV as derived from the temperature dependence of carrier lifetime.