Photothermal Ionization Spectroscopy Research Articles

Kinetics of Formation and Metastability Mechanism for Thermal Donor-Related Defects in Al-Doped Silicon

For almost 40 years, since their discovery in the 1950's [1], thermal donors (TD) in silicon are certainly among the most studied defect systems in solids. Although much effort has been put into the investigation of this defect, the microscopic stucture and the generation kinetics of thermal donors are not fully understood [2]. Even less is known about the TD-related defects, shallow thermal donors (STD), observed for the first time in the mid eighties by photo-thermal ionization spectroscopy [3]. In the Al-doped samples a new member of the STD-family has been found, called the K-donor [4]. It is generated simultaneously with other shallow thermal donors. It has a very similar binding energy, but, in contrary to other STD, it exhibits a metastable behaviour. As it has been found only in Al-doped samples, it is believed to contain aluminium [4]. In this paper we present results of the generation kinetics of the K-donor. The creation process , of the donor is linked to a more general problem of oxygen diffusion in silicon at TD-annealing temperatures. We also attempt to explain the unusual metastable behaviour of the K-donors in terms of an entropy-driven transition. The large entropy change accompanying the metastable transition of the K-donors at low temperatures can in turn be the physical reason of the very efficient diffusion processes leading to the creation of the defect. The K-donor generation experiment was carried out with boron and aluminium doped Si samples (WASO-grade) in numerous annealing steps at 470°C

A copper-related acceptor complex in vacuum grown germanium crystals

A copper-related shallow acceptor complex has been discovered in germanium single crystals grown in vacuum and doped with arsenic and copper. Photothermal ionization spectroscopy of samples quenched from 673 K reveals two sets of hydrogenic lines with ground state binding energies of 9.15 and 10.05 meV. The line intensity ratios between corresponding transitions (1s-np) of the two hydrogenic series follow a Boltzmann dependence. This shows that the two series belong to the same impurity complex with a split ground state. Taking into account the crystal growth conditions together with the changes in the donor concentration deduced from variable temperature Hall effect measurements, we conclude that arsenic and substitutional copper form the new acceptor complex A(Cus,As) since copper is the only fast-diffusing species at this low annealing temperature. This complex is expected to have C3v symmetry in agreement with preliminary piezospectroscopy measurements.

Effects of an external magnetic field on shallow donor levels in semiconductors

The recent progress on investigation of the effects of external magnetic field on shallow impurity centers in semiconductors is discussed. As an example, the shallow donor levels of phosphorus in ultra pure silicon are investigated under magnetic fields and by use of photothermal ionization spectroscopy(PTIS). By selecting the configuration of B∥∥ kββ 〈100〉, the linear and quadratic Zeeman splittings of phosphorus donor states are observed. By selecting the configuration of B∥∥k∥∥ 〈111〉, the anticrossing between hybridized Zeeman states and the intensity evolution of Zeeman transitions near the anticrossing region are observed and measured quantitatively, which, in turn, gives the measurement of wavefunction composition and their evolutions with magnetic fields fox the hybridized Zeeman states of bounded electrons of impurities. An extension of Faulkner's method is made and a variational calculation in the framework of the effective mass approach(EMA) is performed to estimate theoretically the effects of magnetic fields on shallow donor levels and impurity transitions, especially to estimate the wave function compositions of hybridized states. A comparison has been made between the theoretical and experimental results.

Photoconductivity of erbium-doped germanium

We have applied the technique of Photo Thermal Ionization Spectroscopy (PTIS) to the study of an erbium-doped p-Ge epitaxial layer, grown by MBE on an undoped n-type germanium substrate. The Er-doped Ge layer shows continuum photoconductivity response in the far-infrared region extending from 70 cm−1 to 900 cm−1. This type of epitaxial Er-doped Ge layers is a potentially attractive system for photoconductivity detectors of far-infrared radiation. Below 900 cm−1 three acceptor-like charged states can be distinguished with ionization energies of 9, 26.6 and ≥50 meV. Additionally, a study of the photoconductive response of the same sample for radiation from 1000 cm−1 to 10000 cm−1, i.e., for radiation energies well inside the forbidden gap to energies above it, shows a wealth of levels, some of which have previously been associated with erbium.

Temperature-dependent compensation and optical quenching by thermal oxygen donors in germanium.

Photothermal ionization spectroscopy of germanium, doped in the impurity-band conduction range with gallium acceptors and with thermal oxygen donors, reveals that the donors and acceptors compensate each other at temperatures higher than about 5K, but that the impurities coexist as neutral donors and acceptors at lower temperatures.

Observation of confinement effects on acceptors in Si/Si 1−xGe x superlattices

We have used photothermal ionization spectroscopy to study boron acceptors in p-Si/Si 1−xGe x, MBE grown on n-Si substrates. The Si 1−xGe x layers were center-doped with boron in concentrations between 10 17 to 10 18 cm -3. For constant Si layer width, at Si 1−xGe x widths of 100 Å and smaller, bands of acceptor transitions appeared between 120 to 400 cm -1 depending on the width. For 100 Å wells, the observations are explained by the simple hydrogenic model of a confined acceptor of Bastard and the assumption that they form states associated to the heavy hole valence band. This assumption is justified by the well known fact that the valence band is splitted from the light hole band by the stress present in these superlattices. For the 30 Å superlattice, appreciable differences with respect to the Bastard model are observed. This indicate the need to include the finite height of the confining barrier and the effect of the light and heavy hole band states for acceptors confined in wells of 30 Å in width and smaller.

Wave-function mixture and composition for hybridized Zeeman states of P in Si.

We report the experimental determination of the wave-function mixture and composition for hybridized Zeeman states of P in Si and give a related theoretical discussion. The intensity evolution of the Zeeman transitions near the anticrossing region for the isolated P donors in highly pure Si has been measured quantitatively by means of high-sensitivity and high-resolution photothermal ionization spectroscopy. The wave-function compositions and their evolutions with magnetic field for the hybridized Zeeman states of bounded electrons of impurities have been deduced from the measurement. A variational calculation in the framework of the effective-mass approach has been performed to estimate theoretically the wave-function compositions, and a comparison has been made between the theoretical results and experimental ones.

WAVEFUNCTION MIXTURE AND COMPOSITION FOR HYBRIDIZED ZEEMAN STATES OF P IN Si

We report direct experimental determination of the wavefunction mixture and composition for hybridized Zeeman states of P in Si. The intensity evolution of the Zeeman transitions nearby anti-crossing region for the isolated P donors in high pure Si bas been measured quantitatively by use of high sensitive and high resolution photothermal ionization spectroscopy (PTIS); The wavefunction compositions and their evolutions upon magnetic field for the hybridized Zeeman states of bounded electrons of impurities have been deduced from the measurement. A variational calculation under the frame of effective mass approach has been performed to estimate theoretically the wavefunction composition. The results are campare with experimental ones.

Shallow hydrogen-related donors in silicon.

Photothermal ionization spectroscopy on neutron-irradiated and subsequently hydrogen-plasma-treated silicon reveals the existence of new shallow donors. The binding energies of the observed effective-mass-like donors are between 34 and 53 meV. The optical dipole transitions of the different donors are shifted towards higher energies by \ensuremath{\Delta}E=0.1--0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, when deuterium is used in the plasma instead of hydrogen. This isotope shift of the optical dipole transitions between the electronic levels of the defects is direct proof of the incorporation of hydrogen in these defects.

Effects of an external magnetic field on shallow donor levels in semiconductors.

An extension of Faulkner's method for the energy levels of the shallow donor in silicon and germanium at zero field is made in order to investigate the effects of a magnetic field upon the excited states. The effective-mass Hamiltonian matrix elements of an electron bound to a donor center and subjected to a magnetic field B, which involves both the linear and quadratic terms of magnetic field, are expressed analytically and matrices are solved numerically. The photothermal ionization spectroscopy of phosphorus in ultrapure silicon for magnetic fields parallel to the [1,0,0] and [1,1,1] directions and up to 10 T is explained successfully.

Contactless Photothermal Ionization Spectroscopy of Shallow Defects in Semiconductors

Contactless Photothermal Ionization Spectroscopy of Shallow Defects in Semiconductors

Bistable Behaviour of the New Shallow Thermal Donor in Aluminum Doped Silicon

Thermal donors (TDs) in Czochralski-grown silicon belong to the most studied defect systems in solids. They appear after prolonged thermal annealing of the oxygen-rich silicon in the temperature range of about 350-470°C. It is quite well established that the TDs are formed in a process of oxygen clustering, but the formation mechanism itself (simple diffusion cannot account for it), as well as, the influence of codoping remain still unclear. Also despite the extensive research done over last three decades no definitive models exist which could explain their microscopic structure [1-2]. The infrared absorption studies contributed significantly to our present knowledge on properties of TDs [2]. It has been established that two distinct spectral series can be related to TDs, proving them to be double donors (well described by the effective mass theory), with ground state energies of approximately 60 meV (TD0/TD+) and 150 meV (TD+/TD++) below the conduction band. Relatively recent investigations of photothermal ionization spectroscopy [3] revealed a series of lines being a signature of a new single donor species called shallow thermal donors (STDs). They are generated simultaneously to TD and they have ground state energies of about 30 meV.

Application of photothermal ionization spectroscopy to the study of epitaxially grown germanium on silicon

We have applied the technique of photothermal ionization spectroscopy to the study of a 1-μm-thick p-Ge epilayer, grown by molecular beam epitaxy on a n-Si substrate, 500 μm in thickness. The spectra indicate that in the Ge layer there exists a series of charged acceptor defects with an ionization energy continuum starting at 15 meV, an ionization energy somewhat larger than those of the elemental substitutional acceptors. Our results show that photothermal ionization spectroscopy can be applied very advantageously to epitaxial layers of the Si1−xGex alloys that are Ge-like, i.e., for x≥0.85. For these layers, charged impurity centers and defects have their spectral features well separated from those of the Si substrate.

Nitrogen-oxygen complexes in silicon studied by photothermal ionization spectroscopy

High-resolution photothermal ionization spectroscopy has been performed on silicon grown in a nitrogen environment by the Czochralski technique for the first time. Three shallow donors related to the complexes of nitrogen and oxygen impurities D(N-O-3), D(N-O-4), and D(N-O-5) have been observed. Previously unresolved transitions related to the excited states higher than 3p± for D(N-O)s are observed in the spectra as well. The ionization energies of D(N-O)s have been accurately determined as 36.16, 36.41, and 37.37 meV, respectively. In addition, two previously unidentified donor levels found in the absorption spectra of silicon crystal involving nitrogen and oxygen are identified as originating from the splitting of the ground state of phosphorus.

Far-infrared spectroscopy of impurities in semiconductors

Far-infrared spectroscopy of the electronic transitions between bound states of impurities provides a very high resolution technique for studying chemical shifts and thereby identifying residual contaminants. The use of photoconductivity generated within the sample itself, usually by the photothermal mechanism (“photothermal ionisation spectroscopy”), enables very high sensitivity to be achieved even with very thin films or ultrahigh-purity material. The current knowledge about the identity of the residual shallow donors in GaAs, InP, InAs and InSb obtained with this technique is reviewed. With high-purity materials the magneto-optical spectrum of the shallow donors can be particularly rich and more than fifty lines can be observed with both GaAs and InP. Hydrostatic pressure provides a valuable additional experimental parameter in studies of impurities. Not only does the pressure-induced increase in mass improve the resolution of the “fine structure” due to different chemical species but additional states can be introduced into the forbidden energy gap. Results with both InSb and GaAs have shown that generally donors in direct-gap III-V materials may be expected to have three types of state: the familiar gamma-associated donors, localised states with A 1 symmetry which are normally resonant within the conduction band and metastable DX states. Negatively charged shallow donor states (D - states) and “molecular” combinations where the electrons are shared between two or more donor sites have been studied by infrared spectroscopy of III-V materials. These states are important precursors of the metal-insulator transition. Recently there have been a number of studies of impurities within quantum wells and heterostructures. The dependence of impurity energy on distance from the well edge has been established and it has been shown that high concentrations of D - states can be formed by remote deping of the structures.

Electronic properties of hydrogen-related complexes in pure semiconductors

Hydrogen has been shown to activate the neutral impurities carbon, silicon and oxygen in ultra-pure germanium and form shallow level complexes. The double acceptors beryllium and zinc in silicon and germanium, as well as the triple acceptor copper in germanium, can be partially passivated, leading to single hole acceptors. The study of the electronic level spectrum of the single carrier bound to these centers at low temperatures has provided much information on symmetry and composition. Most centers reveal a symmetry axis along [1 1 1] and are static. In some cases hydrogen has been found to tunnel between equivalent real space positions. Photothermal ionization spectroscopy (PTIS) has been the most important tool for the study of the optical transitions of the hole (electron) in these hydrogen containing complexes. This photoconductivity technique combines high sensitivity with high resolution and permits the study of shallow acceptors or donors present at concentrations as low as 108 cm-3. Even lower limits may be attained under favorable circumstances.

Electronic defect characterization in silicon

A brief outline is presented on recent developments in deep-level characterization and identification in semiconductors which have been made possible by the application of methods other than junction space charge measurements. Chalcogens and several transition metals in silicon are used as examples in order to show how important parameters and properties of defects can be revealed by using spectroscopic methods. One of the methods, namely photothermal ionization spectroscopy is discussed in more detail.

Photothermal ionization spectroscopy for shallow impurities in ultra-pure silicon

We report the comprehensive results obtained in our group and last few years for the shallow impurities in ultrapure silicon by use of photothermal ionization spectroscopy. The new results reported here include the discovery and investigation of new shallow impurity centers in Si, the detection for the compensation of different types of impurities, the accurate determination for the spin-orbit splitting Δ. of valence band for Si, and the phonon duplicates and Fano resonance for the transitions of shallow impurities in Si. In addition it is also shown experimentally that the sensitivity of the photothermal ionization spectroscopy as used for detecting the concentration of shallow donors in Si can reach as high as 108 cm−3, much higher than that reported in the literatures up to date, and line width for the sharpest spectral lines in the spectrum is about 0.08 cm−1, that is, 10 μeV.

New shallow donors in high-purity silicon single crystal

High-resolution photothermal ionization spectroscopy has been performed on n-type high-purity silicon crystals. Two new shallow donors with binding energies of 36.61 and 36.97 meV, respectively, are observed for the first time, in addition to lithium (Li), lithium-oxygen complex [D(Li,O)], phosphorus (P), and arsenic (As) donors. These new shallow donors show similar spectral features to Li, D(Li,O), P, and As, which implies that they are also hydrogenic shallow donors. These new complex centers are probably generated during crystal growth. In addition, previously unresolved transitions related to very high excited states of phosphorus as well as the above-mentioned new shallow donors, have also been observed.

Spectroscopic studies of the influence of oxygen partial pressure on the incorporation of residual silicon impurities in vapor-phase epitaxial gallium arsenide

The incorporation of residual shallow impurity species, in particular Si, in both chloride and hydride vapor-phase epitaxial GaAs layers grown with oxygen intentionally injected into the source zone has been studied. Photothermal ionization spectroscopy and photoluminescence show that both the Si donor and acceptor concentrations are significantly reduced by the oxygen. The observed reduction of carrier concentration and the increase in Hall mobility with increasing oxygen partial pressure are mainly due to the reduction of Si impurity concentration. Although oxygen can form deep levels as indicated by the 1.4889-eV bound-exciton line in photoluminescence, the main effect of oxygen injection is to improve the material purity by suppression of the incorporation of residual Si.