Photothermal Ionization Research Articles

Deep Traps in AlN Hydride Vapor Phase Epitaxy Film on Low-Temperature AlN/Sapphire

Deep trap states were examined in c-plane, Al-polar AlN epilayers, deposited via metal organic chemical vapor deposition on 270 nm thick AlN buffer layers on sapphire substrates. Contacts were created using e-beam deposited Ni through a shadow mask, with I-V characteristics revealing a trapped limited current (TLC) regime and voltage-dependent hysteresis upon light-emitting diode illumination. Thermally stimulated current and photothermal ionization current spectroscopy measurements demonstrated a prominent trap activation energy of approximately 0.75 eV and additional trap energies of 0.6, 0.4, 0.25, 1.05, and 1.1 eV. The observed differences in photocurrent responses between forward and reverse biases suggest that forward bias induces electron trapping at deeper levels, influencing the TLC behavior. Comparisons with bulk n-type AlN crystals from previous studies show similarities in deep trap spectra, suggesting commonality in trap characteristics across different AlN samples.

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.

Model for photothermal ionization and molecular recombination during pulsed ablation of polyethylene

The surface erosion of polyethylene is studied under conditions characteristic of extremely high-rate heating expected in laser-driven and high energy density experiments. A coupled model for photoionization and volumetric photothermal degradation of polyethylene is developed that includes finite rate chemistry and molecular recombination of radicals. First, the model is used to explore the evolution of radicals and hydrocarbons during isochoric heating, for which it is found that polyethylene is not in chemical equilibrium for rates >108K/s. Then, the model is used to explore the cooperativity between photoionization and photothermal ionization during one-dimensional ablation from a pulsed heating source, accounting for coupled energy deposition, thermokinetics, thermochemistry, hydrodynamics, mechanics, and thermal conduction in the ALEGRA multi-physics code. The ablation depth per energy pulse is found to agree with an analytical model for instantaneous energy deposition and absent molecular recombination or photoionization. Otherwise, molecular recombination of radicals reduces the depth of ablation per pulse, owing to thermal stabilization on forming branched molecules. Radical formation through photoionization can be compensated for by recombination, but it accelerates ablation at high enough photoionization rates. Finally, the influence of molecular recombination on the ablation depth is found to decrease for long pulses, owing to decreased recombination reaction rates at lower vapor densities near the ablated surface. These and other findings highlight competition between the processes of scission, recombination, and photoionization during pulsed ablation of polyethylene.

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.

Quenching of the Sm2+ luminescence in NaMgF3:Sm via photothermal ionization: Alternative method to determine divalent lanthanide trap depths

Photoluminescence and radiophotoluminescence measurements were performed on the promising dosimeter compound NaMgF3 doped with Sm at different temperatures. Quenching of the Sm2+ photoluminescence during continuous 448 nm optical stimulation into the Sm2+ 4f 55d1 level was observed at high temperatures. We attribute this to photothermal ionization from the Sm2+ 4f 6(7F0) ground state to the NaMgF3 conduction band followed by nonradiative decay to the Sm2+ ground state. This was confirmed by X-ray-induced radiophotoluminescence, Sm3+ + e− → Sm2+, where the valence conversion was reversed by simultaneous thermal and optical stimulation. From these measurements, we deduce that the energy from the Sm2+ 4f 55d1 level to the conduction band is 1.2 eV and the energy from the Sm2+ 4f 6(7F0) ground state to the conduction band is approximately 4.0 eV. Thus, we provide an alternative method to determine the trap depths of divalent lanthanides in insulators, which is of critical importance in the development of commercially relevant luminescent materials.

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.

Luminescence emission from metastable Sm2+ defects in Y PO4:Ce,Sm

Ce3+ and Sm3+ both form stable defect centres in Y PO4, and their emission properties are well known. However, by irradiating co-doped Y PO4:Ce,Sm with x-rays or UV light the charge states of the defects can be modified to become Ce4+ and Sm2+, which are metastable, and their behaviour acts as a model system for understanding carrier dynamics in charge storage phosphors. Here we report on the luminescence emission behaviour of the Sm2+ defects that can be observed after x-irradiation. Under suitable excitation conditions, emission from both the stable Sm3+ and metastable Sm2+ can be monitored simultaneously. The Sm2+ luminescence is found to be comprised of a series of narrow lines in the energy range 1.5–1.8 eV, identifiable as internal 4f–4f transitions, accompanied by a series of phonon replicas (phonon energy, 20.4 meV). The intensity of the metastable Sm2+ emission increases in proportion to the x-irradiation time as their population is increased; under 1.92 eV laser stimulation, the PL is found to fully quench at temperatures above 150 K due to photo-thermal ionization of the defect.

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.

Photo-thermal ionization spectroscopy of shallow acceptors in high purity germanium

The results of high sensitivity photo-thermal ionization spectroscoic (PTIS) investigation of shallow acceptors in high purity p-type germanium sample with room temperature resistivity of about 50Ω·cm, at temperature above liquid helium by Fourier transform spectrometer and associated magneto-optical measurement systems, are reported. The optimum photo-thermal ionization temperature range for the shallow impurities in high purity germanium is determined experimentally. At temperatures within this range, the PTIS spectra of this sample were measured. Boron and aluminum are found to be the main shallow acceptors in the sample. The causes of line splits of spectra, rapid recombination of compensating impurity and random strain, are analyzed and discussed.

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.

Optical properties of nanocrystalline diamond thin films

The optical properties of nanocrystalline diamond films grown from a hydrogen-rich CH4∕H2 gas phase by hot filament chemical vapor deposition, as well as from an argon-rich Ar∕CH4 gas phase by microwave plasma enhanced chemical vapor deposition, are reported. The influence of nitrogen incorporation on the optical absorption is investigated. The diamond films are characterized by photothermal deflection spectroscopy and temperature dependent spectrally resolved photoconductivity. An onset of absorption at about 0.8eV in undoped films is attributed to transitions from π to π states introduced into the band gap by the high amount of sp2 bonded carbon at the grain boundaries. Incorporation of nitrogen leads to a strong absorption in the whole energy spectrum, as a result of the increasing number of sp2 carbon atoms. The effect of surface states has been observed in the high energy region of the spectrum. Transitions to the conduction band tail and photothermal ionization processes account for the observed onset at 4.4eV. Photocurrent quenching at about 3.3eV is observed in the case of samples grown from a hydrogen-rich CH4∕H2 gas phase.

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.