Dynamics of nonequilibrium charge carriers in n- and p-type germanium

In recent years, THz sources which deliver short pulses in the picosecond range with relatively high peak power have become more widespread. These short-pulse THz sources require fast broadband detectors with a large dynamic range. Germanium (Ge) photoconductive detectors have the potential to comply with the requirements necessary for these sources. The detection mechanism is based on the photo ionization of so called shallow centers. These centers create hydrogen-like energy levels in the bandgap. THz photons can be absorbed due to their low binding energy. On that account these detectors operate at liquid Helium temperatures. Examples for shallow centers are p-type dopant antimony (Sb) and n-type dopant gallium (Ga). Thus far, the shortest response time of a Ge photoconductive detector at THz frequencies was obtained using highly compensated (32%-52%) p-Ge:Ga:As:Se with a moderate acceptor concentration of (1−2)×1E15 cm−3 and a total compensating donor concentration of (5.2−14)×1E14 cm−3. The response time to a 100ps-long THz pulse (2.5 THz) of a free electron laser (FEL) was about 3ns. The recovery speed of the detector material is, however, fundamentally limited only by the band-to-impurity recombination time down to the impurity ground state. The purpose of this work was to determine these fundamental recombination times in differently doped Ge crystals. This has been carried out for the first time in n- and p-type Ge with a dedicated pump-probe-experiment using the FEL FELBE of the Helmholtz-Zentrum Dresden-Rossendorf. Moreover, the lifetimes of some excited states have been ascertained. For these measurements special Ge samples have been fabricated by the Leibniz-Institute for Crystal Growth in Berlin. Mainly these samples can be subdivided into two groups, i.e. moderately doped Ge (n-Ge:Sb, n-Ge:As und n-Ge:Ga, ≈1E15 cm−3) and heavily doped and highly compensated Ge (p-Ge:Ga:Sb and n-Ge:Sb:Ga, with NGa , NSb ≈1E16 cm−3 and |N_ Ga−N_Sb|≈1E15 cm−3). The recombination time was shown to depend on the photon flux density of the FEL, typical values are a few ns. With supporting measurements of the temperature dependence on the recombination times an excitation and recombination scheme in p- and n-Ge is proposed. This scheme includes intraband processes. The time constant of the intraband relaxation is independent on the doping level and on the photon flux density for the investigated samples and amounts to ≈200ps. The refined cascade model is commonly used to describe the capture of non-equilibrium carriers by ionized centers. It predicts two regimes. In the case of low photon flux density, i.e. low ionization density, the recombination time has a different dependence on the ionization level in the sample compared to a high ionization density at large low photon flux densities. Both dependences could be identified by the experiment. This supports the refined cascade model in its two limiting cases. The determination of the recombination times in highly doped and highly compensated p- and n-Ge delivered values between a few ps up to ≈300ps . Despite the same net concentration this is one to two orders of magnitude shorter compared to moderately doped Ge. Due to the high doping the recombination times cannot be explained by the refined cascade model. With the highly doped and highly compensated material a photoconductive detector has been fabricated and characterized. To a 10ps-long FEL pulse it was possible to obtain a detector response with a full width half maximum of ≈160ps. This is the fastest response ever demonstrated with an extrinsic Ge photoconductor and it is about 20 times faster than the previous record.In den letzten Jahren ist die Erzeugung von THz-Pulsen im Femto- bzw. Pikosekundenbereich mit hohen Spitzenleistungen stetig verbessert und gebräuchlicher geworden. Diese Strahlungsquellen erfordern schnelle und spektral breitbandige Detektoren mit einem hohen dynamischen Bereich. Extrinsische Germanium-Photoleiter (Ge) haben das Potential diese Lücke zu schließen. Der Detektionsmechanismus basiert auf der Photoionisation von sogenannten flachen Störstellen. Diese erzeugen wasserstoffartige Energieniveaus in der Bandlücke. Aufgrund der geringen Bindungsenergie der Störstellen können THz-Photonen detektiert werden. Dafür müssen diese Detektoren jedoch bei flüssig Helium-Temperaturen betrieben werden. Zu den flachen Störstellen gehören beispielsweise das p-dotierende Gallium (Ge:Ga) und das n-dotierende Antimon (Ge:Sb). Die bislang kürzeste Antwortzeit eines solchen Detektors wurde mittels einer hoch kompensierten p-Ge:Ga:As:Se-Probe mit einer moderaten Akzeptorkonzentration von (1−2)×1E15 cm−3 und einer Gesamtkonzentration an Donatoren von (5,2 − 12)×1E14 cm−3 erreicht. Die Antwortzeit auf einen 100ps-langen THz-Puls (2,5THz) eines Freie-Elektronen-Lasers (FEL) betrug etwa 3ns. Auf fundamentalem Niveau ist die Antwortzeit limitiert durch die Rekombination freier Nichtgleichgewichtsladungsträger mit ionisierten Störstellen. Das Ziel der Arbeit bestand darin, diese fundamentalen Rekombinationszeiten in verschieden dotierten Ge-Kristallen zu bestimmen. Dies wurde hier erstmalig für n- und p-Ge mit einem dedizierten Ein-Farben-Pump-Probe-Experiment mit dem Freie-Elektronen-Laser (FEL) FELBE des Helmholtz-Zentrums Dresden-Rossendorf durchgeführt. Außerdem wurden die Lebensdauern einiger angeregter Zustände bestimmt. Für die Messungen wurden Ge-Proben am Leibniz-Institut für Kristallzüchtung in Berlin hergestellt. Die Proben teilen sich auf in moderat dotiertes Ge (n-Ge:Sb, n-Ge:As und n-Ge:Ga, ≈1E15 cm−3) und hoch dotiertes und hoch kompensiertes Ge (p-Ge:Ga:Sb und n-Ge:Sb:Ga, mit N_Ga , N_Sb≈1E16 cm−3 und |N_Ga−N_Sb|≈1E15 cm−3). Die Rekombination hat eine Zeitkonstante, die von der Photonenflussdichte des FELs abhängt und typischerweise einige ns beträgt. Mit der zusätzlichen Bestätigung durch temperaturabhängige Messungen wurde ein Anregungs- bzw. Rekombinationsmechanismus in p- und n-Ge vorgeschlagen, der Intrabandprozesse berücksichtigt. Die Zeitkonstante der Intrabandrelaxation ist unabhängig von der Dotierung und der Photonenflussdichte für die Proben und beträgt ≈200ps. Das erweiterte Kaskadenmodell dient der Beschreibung des Einfangs freier Ladungsträger durch ionisierte Störstellen und sagt zwei Regime voraus. Im Bereich geringer Photonenflussdichten, d. h. geringer Ionisierungsdichten weist die Rekombinationszeit eine andere Abhängigkeit vom Ionisierungsgrad auf als im Bereich sehr hoher Ionisierungsdichten. Diese Abhängigkeit konnte für beide Bereiche nachgewiesen werden. Das erweiterte Kaskadenmodell kann in den Grenzfällen damit als bestätigt angesehen werden. Die Bestimmung der Rekombinationszeiten in hoch dotiertem und hoch kompensiertem p- und n-Ge lieferte Werte von einigen ps bis maximal ≈300ps . Diese waren bei gleicher Nettokonzentration um ein bis zwei Größenordnungen kleiner als bei moderat dotiertem Ge. Aufgrund der hohen Dotierung lassen sich die Rekombinationszeiten nicht mehr durch das Kaskadenmodell beschreiben. Mit dem kompensierten Material wurde ein photoleitender Ge-Detektor hergestellt und charakterisiert. Es ist gelungen etwa 10ps-lange FEL-Pulse mit einer Halbwertsbreite von ungefähr 160ps zu messen. Dies ist die kürzeste jemals gezeigte Antwortzeit eines Ge-Photoleiters und ist etwa um den Faktor 20 schneller als der bisherige Rekord

Identification of unknown substances by terahertz spectroscopy and multivariate data analysis

The identification of various substances by multivariate data analysis of terahertz transmittance spectra is demonstrated. Transmittance spectra were obtained by the use of a Fourier transform infrared spectrometer. By means of principal component analysis and partial least squares regression, the spectral data were analyzed in order to identify substances and mixtures of several substances. With only three principal components, detection and identification of substances are possible with high accuracy. Using these methods, concentration ratios of substances in mixtures of two substances can be determined with an accuracy of 10 %. It is shown that the method is robust against disturbances in the spectra such as standing waves. This is particularly important for practical applications

Identification of Substances by THz Spectroscopy and Multivariate Analysis

The identification of different substances based on terahertz transmittance spectra and multivariate analysis (MVA) is demonstrated. Transmittance spectra are obtained by the use of a Fourier transform infrared spectrometer (FTIR). By means of principal component analysis (PCA) and partial least squares regression (PLS) the spectral data were analyzed in order to discriminate and to identify the substances from each other

Determination of the Timing Jitter of THz-Synchrotron Radiation by a Cross-Correlation Technique

The objective of this study is to determine the timing jitter of THz synchrotron radiation. The jitter at the beamline is a composition of several instabilities related to the electron beam and the beam optics. By comparing pulses through correlation techniques it is possible to determine the jitter without the need for direct pulse timing measurements

THz autocorrelation measurements at the Metrology Light Source

We have developed a flexible Martin-Puplett Interferometer setup utilizing broad band coherent terahertz (THz) synchrotron radiation provided by the Metrology Light Source (MLS). In order to obtain frequency resolved measurements we combined the Martin-Puplett setup with a Fourier transform spectrometer (FTS). Frequency resolved autocorrelation measurements for beam diagnostics will be presented

Time-resolved electronic capture in n-type germanium doped with antimony

The low temperature (T=5–40 K) capture of free electrons into hydrogenlike antimony centers in germanium has been studied by a time-resolving experiment using the free electron laser FELBE. The analysis of the pump-probe signal reveals a typical capture time of about 1.7 ns that decreases with pump energy to less than 1 ns while the number of ionized donors increases. The dependence on the pump-pulse energy is well described by an acoustic phonon-assisted capture process. In the cases when (i) a significant number of the electrons is in the conduction band (flux densities larger than 5× 1E25 photons/(cm2 s), (ii) the lattice temperature is above ~20 K, or (iii) a static electric field above ~2V/cm is applied to the crystal, the pump-probe technique reveals an additional intraband relaxation process with a characteristic time of ~100 ps, which is much shorter than that of the capture of free electrons into the antimony ground state

Terahertz absorption and emission upon the photoionization of acceptors in uniaxially stressed silicon

Experimental data on the spontaneous emission and absorption modulation in boron-doped silicon under CO2 laser excitation depending on the uniaxial stress applied along the [001] and [011] crystallographic directions are presented. Room-temperature radiation is used as the probe radiation. Low stress (less than 0.5 kbar) is shown to reduce losses in the terahertz region by 20%. The main contribution to absorption modulation at zero and low stress is made by A+ centers. Intersubband free hole transitions additionally contribute to terahertz absorption at higher stress. These contributions can be minimized by compensation

Time-resolved electronic capture in germanium doped with hydrogen-like impurity centers

The availability of intense short-pulsed THz radiation from sources such as free electron lasers (FELs) or synchrotrons demands broad-band detectors with very short response times. This triggered a renewed interest in fast germanium (Ge) detectors. The fastest operation of Ge detectors demonstrated in the THz region of the electromagnetic spectrum so far showed an about 2-ns long decay time using highly compensated neutron transmutated p-Ge:Ga:As:Sb. The short-pulse narrow-band FEL radiation allows studying impurity photoconductivity kinetics and provides information important for optimizing the speed of response of extrinsic photoconductors. The capture of free holes and electrons in Ge doped by gallium (Ga) or antimony (Sb) has been studied by a time-resolved pump-probe experiment with the FEL FELBE at the HZDR. For Ga acceptors the relaxation times decrease with increasing pump power from approximately 3 ns to 1 ns (2 ns and 1 ns for Sb donors, respectively). The results support the development of fast photoconductive detectors in the THz region of the spectrum