Photothermal Ionization Spectroscopy Research Articles

Properties of a highly compensated high-purity germanium

The electrical and optical properties of a compensated high-purity germanium (HPGe) single crystal was investigated using various characterization techniques. Aluminium, boron, and phosphorus were the major residual shallow-level impurities identified by photothermal ionization spectroscopy (PTIS). Hall effect measurements performed at low temperatures (77 K) along the growth length reveal that the crystal is p-type at the top and bottom, while n-type in the middle part with a net carrier concentration in the range of 1010–1011 cm−3. The obtained very high resistivity (2.3 ⨯ 108 Ω·cm) at 91 K in the temperature-dependent Hall measurements (TDH) at the bottom part of the crystal indicates a high level of compensation (84%) of charge carriers by deep-level impurities or defects with an activation energy near 0.195 eV. The highest hole mobility(μp=46700cm2/V·s)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$({\\mu _p} = ~46700\\,{\\rm{c}}{{\\rm{m}}^2}\\,/{\\rm{V}}\\cdot{\\rm{s}})$$\\end{document} at 77 K was obtained at the top part, which is moderately compensated. The carrier lifetimes of the grown HPGe crystals were measured using microwave-detected photoconductivity (MDP) at room temperature. The average carrier lifetime decreases from the top to bottom part of the crystal from 130 to 60 µs, which is less than that of the usually observed values in HPGe crystals due to a strong compensation.

Technology development of high purity germanium crystals for radiation detectors

Abstract High Purity Germanium (HPGe) crystals are indispensable for radiation detection and fundamental research. Among other applications they will be used in the Large Enriched Germanium Experiment for Neutrinoless double beta Decay (LEGEND) experiment for the search of neutrinoless double beta decay of 76 Ge. Ongoing research activities are done in collaboration with members of the existing GERDA experiment. In this context the Leibniz-Institut fur Kristallzuchtung (IKZ) is engaged in the development of HPGe Czochralski crystal growth including purification and characterization of the material. The Ge starting material is purified in a horizontal multi-zone furnace. Two inch Ge single crystals were grown by the Czochralski method and analyzed with respect to electrically active impurities by means of Hall-effect measurements and Photo-thermal Ionization Spectroscopy. The dislocation density is assessed by combining defect etching and optically microscopy imaging. Lateral Photovoltage Scanning is applied to visualize doping striations, which bear information on the shape of the solid–liquid phase boundary during growth.

Wide tunability and electron transfer in GaAs/AlGaAs quantum well photodetector by magnetic field

One strategy for terahertz (THz) detection in GaAs/AlGaAs quantum well photodetectors under the magnetic field is reported. The THz detection begins to operate after the normally empty hydrogenic donor ground states in the AlGaAs barriers become populated by electrons transferred from the GaAs wells. Through the Landau quantization arising from a perpendicular magnetic field, we achieved the electron transfer from subband Landau levels in the GaAs wells at liquid helium temperature when the magnetic field reaches a certain threshold. One detector based on this strategy exhibited a dramatic range of frequency tunability of 3.20–6.13 THz. Our photothermal ionization spectroscopy measurements show quantitative agreement with the theoretical calculation of intradonor transition energies, verifying the origin of the strongly enhanced frequency tunability from the Zeeman behavior of transferred electrons in the AlGaAs barriers. This finding is useful for exploring magneto-optical effects and realization of wide tunability in THz photodetectors.

Study on the Properties of High Purity Germanium Crystals

In the crystal growth lab of South Dakota University, we are growing high purity germanium (HPGe) crystals and using the grown crystals to make radiation detectors. As the detector grade HPGe crystals, they have to meet two critical requirements: an impurity level of ∼109 to 10 atoms /cm3 and a dislocation density in the range of ∼102 to 104 / cm3. In the present work, we have used the following four characterization techniques to investigate the properties of the grown crystals. First of all, an x-ray diffraction method was used to determine crystal orientation. Secondly, the van der Pauw Hall effect measurement was used to measure the electrical properties. Thirdly, a photo-thermal ionization spectroscopy (PTIS) was used to identify what the impurity atoms are in the crystal. Lastly, an optical microscope observation was used to measure dislocation density in the crystal. All of these characterization techniques have provided great helps to our crystal activities.

Calibration of the photoluminescence technique for measuring concentrations of shallow dopants in Ge

A systematic study of the photoluminescence (PL) from excitons bound to shallow donors or acceptors in Al-, As-, B-, Ga-, In-, P-, and Sb-doped Ge is presented. The results of the PL measurements are correlated with those from photothermal ionization spectroscopy and Hall effect measurements. The dissociation energy of the excitons is shown to satisfy Haynes rule. A calibration is presented, which allows a determination of the donor and acceptor concentrations from their respective bound exciton intensities related to the free exciton intensity.

Long Lifetime State of Shallow Donor Centres for Silicon Based THz Sources

Impurity centres in high purity silicon with ionization energies 38.32 and 40.09meV are observed by photothermal ionization spectroscopy measurements. Their typical shallow donor characteristics are approved by good agreement with theoretic evaluation under varying magnetic fields. The 2p0 state lifetime of one donor centres are more than 3 times longer than that of phosphorus in the same silicon sample. Compared to phosphorus which has been already successfully exploited to produce silicon-based THz radiations, this shallow donor centre has a pronounced enhancement on lifetime and quality factor of population inversion level. Hence, silicon with this donor centre would be a potential excellent candidate to develop improved silicon-based THz sources.

Compensating impurity in high purity silicon single crystal investigated by photo-thermal ionization spectroscopy

The optimum photo-thermal ionization range near liquid helium temperature for the shallow impurities in n type high purity silicon single crystal was determined experimentally. At temperature within this range, the high resolution photo-thermal ionization spectra (PTIS) of this sample were measured. Positive PTIS responses from the majority residual impurity phosphor (P) and compensating impurity boron (B) were observed by simultaneously irradiating the sample with far infrared and band-edge light. Under action of applied external magnetic fields, the positive response from compensating impurity B changes to negative response. The posibility of these phenomena being the result of temperature effect was excluded, which revealed the deficiency of Darken model and supported the model of rapid recombination of minority carriers for PTIS response of compensating impurity in elemental semiconductors.

New shallow donor centres in high-purity Czochralski-grown silicon single crystals

Two new shallow donor centres NSD(i) and NSD(ii) with binding energies of 38.32 meV and 40.09 meV, respectively, were observed in n-type high-purity Czochralski-grown silicon single crystals by means of high-resolution photothermal ionization spectroscopy. Their spectral features and the temperature dependence of the intensity of their spectral lines imply that they are two new independent hydrogenic shallow donors. Low magnetic field investigations were performed to support our statement. The existence of the new donor centres seems to be related to the micro-defects in the sample.

Investigation of hydrogenated CVD diamond films by photo-thermal ionization spectroscopy

Different hydrogenated CVD diamond films as well as bulk IIb and IIa diamonds were investigated by means of photo-thermal ionization spectroscopy. All samples show a new photocurrent signal at temperatures lower than 120–20 K depending on the sample with a threshold around 100 meV what suggests a presence of shallow levels on the hydrogenated diamond surface. The photocurrent spectrum due to these shallow levels consists of features similar to the two-phonon absorption in diamond. Influence of annealing in air on the photocurrent behavior was investigated as well.

The phosphorous level fine structure in homoepitaxial and polycrystalline n-type CVD diamond

The application of very sensitive photocurrent-based spectroscopic techniques have led to the detection of new levels for the electronic structure of the phosphorous donor in n-type CVD diamond. By combining quasi-steady-state photocurrent measurements (PC), photothermal ionisation spectroscopy (PTIS) and the highly sensitive Fourier transform photocurrent spectroscopy (FTPS) technique at different temperatures, ranging from liquid nitrogen temperature to 170 K, the resulting spectra point to a richer structure than assumed up to now. This is the consequence of the improved sample quality over the last years, opening up to a much larger attainable doping window. By using doping levels, ranging from 10 19 cm −3 down to 10 16 cm −3 on {111}-oriented Ib HPHT substrates, still giving rise to measurable n-type conductivity, spectra showed less line broadening and more fine structure. Finally, the results will be compared with spectra measured on active P-doped polycrystalline n-type films.

A quantitative study of the boron acceptor in diamond by Fourier-transform photocurrent spectroscopy

Fourier-transform photocurrent spectroscopy (FTPS) was used as a sensitive spectroscopic method to study boron doping in diamond layers, deposited by microwave plasma enhanced chemical vapor deposition (CVD). Results were compared to boron doped single crystals, synthesized at high pressure and high temperature. Introduction of boron increases the photosensitivity of diamond; the measured spectrum of excited states of the boron acceptor depends both on the crystal perfection and the boron concentration. The spectrum of excited states is a clear fingerprint of the presence of substitutional boron (up to 31 absorption lines have been identified). A quantitative assessment of the substitutional boron concentration is possible and the sensitivity of FTPS in combination with photothermal ionization spectroscopy (PTIS) for boron detection in diamond is estimated to be better then 1 part per billion (ppb).

Zeeman spectroscopy of Be impurity in GaAs to [formula omitted

Absorption measurements have been made in fields to 30 T of the far-infrared optical transitions associated with the Be impurity in GaAs. The order and magnitude of the splitting of the ground state has been clarified by low-field (to 6 T ) photo-thermal ionisation spectroscopy measurements of the C line. In light of the new high-field data the G line is now believed to comprise two unresolved components. At high magnetic field (above 25 T ) a new feature appears which increases in energy with field at a much greater rate than the other transitions; this is thought to originate in valence-band Landau levels.

Magnetospectroscopy of Be in GaAs

Far-infrared absorption measurements have been made of the Lyman series of the acceptor Be in GaAs up to magnetic fields of 30 T, supplemented by photothermal ionization spectroscopy measurements to 6 T. The results confirm and greatly extend previous work and reveal several phenomena. The strongest component of the G line moves to higher energies and increases in intensity with field. Around 20 T the field dependence of the transition displays a marked kink. At higher fields the dependence of energy on field is linear. This behavior is attributed to unresolved components that change in relative intensity with field and leads to a reappraisal of the identity of the low-field transition. The D line is followed to high fields allowing an accurate determination of the splitting of the field-induced components of the second excited state. Photothermal ionization spectra for the C line permit the determination of the splitting of the field-induced components of the ground state and of the third excited state. At high magnetic field an absorption appears which increases in energy with field at a much greater rate than any of the lines mentioned above. It is thought to originate in the valence-band Landau levels.

Magnesium Acceptor Energy Levels in Cubic GaN

Intra-impurity transitions of the Mg acceptor in cubic phase GaN were measured with the use of photothermal ionization spectroscopy. Apart from the photoionization band several sharp features were detected, related to internal Mg-acceptor transitions. The transitions were identified with the help of effective-mass model calculations involving light- and heavy-hole as well as spin-orbit split-off bands. Transitions to resonant states, associated with the spin-orbit split-off valence band, were also identified. The determined hole binding energy of the Mg acceptor in zinc-blende GaN is 236 ′ 1 meV.

Phonon-assisted electronic transitions in phosphorus-doped n-type chemical vapor deposition diamond films

One year ago we published the first results on the electronic structure of the P-level in 1000 ppm PH 3/CH 4 doped {111}-oriented n-type diamond films, using the quasi-steady-state photocurrent technique (PC) and photothermal ionization spectroscopy (PTIS). In this work we have extended our measurements at various temperatures (4.2–77.4 K) to samples with various doping levels (100, 500 and 1000 ppm PH 3/CH 4). This allowed us to obtain more precise results for the electronic structure of the phosphorus defect in homoepitaxial n-type CVD diamond films, making use of the 155 meV LO-phonon to explain the oscillatory photoconductivity. These results are confirmed by the PTIS maxima and Fourier transform infra-red (FTIR) data. In addition we present first measurements on a 2000-ppm doped {100}-oriented sample.

Gas source molecular beam epitaxy of InP-based microstructures: material growth, characterization and device applications

InP epitaxial layers and InGaAs/InAlAs/InP modulation doped heterostructures were grown by gas source molecular beam epitaxy, in which arsenic and phosphorus beams were obtained by thermal cracking of AsH 3 and PH 3. High purity InP layers have been obtained by optimizing growth parameters. To identify the species of impurity and defect in high quality InP layers, low temperature photoluminescence (PL) and photothermal ionization spectroscopy measurements were carried out. The residual donors were determined to be silicon and sulfur. To obtain a deep insight to the electronic subband structure of the two-dimensional electron gas (2DEG) system, Shubnikov-de Haas, quantum Hall effect measurements and field-dependent cyclotron resonance measurements were conducted. The electron density on each subband in 2DEG was determined to be 2.84×10 12 and 3.4×10 11 cm −2, respectively. The effective masses of the two subbands are (0.061±0.001) m 0 for the zero subband and (0.049±0.001) m 0 for the first subband. An optoelectronic integrated circuit (OEIC) structure consisting of high electron mobility transistor (HEMT) and metal semiconductor metal photo detector (MSM-PD) was achieved by gas source molecular beam epitaxy with one growth run. The responsibility of MSM-PD is 0.5 A/ W. DC transconductance of an InGaAs/InAlAs HEMT with 1.5-μm gate length is 305 mS mm −1; the maximum saturation current is 350 mA mm −1 and the pinch-off voltage is −1.4 V. The gain of the HEMT amplifier is 14 dB. The integrated photoreceiver could successfully operate at a data rate of 622 Mbit s −1.

Shallow Nitrogen Donor States in 4H-SiC Investigated by Photothermal Ionization Spectroscopy

Shallow Nitrogen Donor States in 4H-SiC Investigated by Photothermal Ionization Spectroscopy

Photothermal ionization spectroscopy of shallow nitrogen donor states in 4H–SiC

Photothermal ionization spectroscopy (PTIS) measurements were carried out on a free-standing, high purity and high quality 4H–SiC epitaxial layer at various temperatures. The two step photothermal ionization process is clearly reflected in the temperature dependence of the photoconductivity. The PTI spectrum at a temperature of 25.6 K exhibits one order of magnitude higher energy resolution than the infrared absorption spectra of 4H–SiC bulk material. It reveals five strong, well resolved electronic transition lines associated with the shallow nitrogen donor. The ionization energy of the shallow nitrogen donor is deduced to be 60.2±0.5 meV based on experimental results. Furthermore, PTI magnetospectroscopy measurements were performed to investigate the symmetry properties of these transitions in Faraday configuration. No linear Zeeman splitting is observed, however, these lines show a diamagnetic shift. It indicates that the excited states of the shallow nitrogen donor are nondegenerate at zero magnetic field, which is consistent with the fact that the effective mass tensor of 4H–SiC has three different diagonal components.

Temperature dependent spectroscopic study of the electronic structure of phosphorus in n-type CVD diamond films

Abstract In this work we used the quasi-steady-state photocurrent method and photothermal ionisation spectroscopy (PTIS) to study the electronic structure of the P-related level in n-type phosphorus doped CVD diamond. Previously, we reported the existence of two optically active defect levels, labelled XP1 and XP2, in the bandgap of these P-doped layers. XP2 appeared to be only present in films showing high resistivity. Here we present a detailed PTIS and photocurrent study of the photoionisation cross-section, from liquid helium up to room temperature. At low temperatures, samples containing only the XP1 defect show a phonon-induced oscillatory photoconductivity, giving information about the electronic structure of this defect. Additionally, with PTIS a sharp maximum was detected around 565 meV, originating from a thermal promotion of charge carriers from an excited state of the phosphorus-related level into the conduction band. We also discuss different models for the optical ionisation cross-section of the P-related level (e.g. shallow level, deep level).

Study of bound exciton excited state structure using photothermal ionisation spectroscopy

Photothermal ionisation spectroscopy (PTIS) has been used for many years to study the electronic structure of dopants in semiconductors, but to our knowledge it has been applied only once to the study of isoelectronic bound exciton (BE) states. The absence of any substantial literature on the subject raises the question as to whether some fundamental factors preclude the observation of the sharp line transitions from the exciton excited states. We have attempted to measure PTIS effects in a range of BE systems in silicon, but only certain systems prove amenable to the technique. In addition to data on the C-line (which has been reported in another study), PTIS spectra of BE complexes at two other well-studied centres in silicon are reported, which are generally comparable to published photoluminescence excitation (PLE) and absorption spectra. The intensities of the lines do not show, in all cases, the expected systematic temperature dependence, demonstrating extreme temperature sensitivity, with thermal activation energies different to those obtained from photoluminescence (PL) temperature dependence data. Finally, we comment on the potential of the technique for the study of excited states of BE in silicon.