Oxygen-doped Germanium Research Articles

Infrared analysis of the precipitated oxide phase in silicon and germanium

Oxygen-doped germanium samples have been subjected to a prolonged precipitation treatment in the 560–620 °C temperature range. Broad infrared absorption bands with maxima at 865 and 884 cm −1 which develop proportionally with the loss of interstitial oxygen are observed and are attributed to germanium oxide precipitates. Secondary peaks appear at 675 and 958 cm −1. The broad absorption bands are analyzed using a procedure which was previously applied to SiO x precipitates in silicon. Similarly as in silicon, the absorption in germanium may be fitted assuming the presence of polyhedral and platelet GeO x precipitates, with relative occurrence depending on the details of the thermal treatment.

Interacting quantum rotors in oxygen-doped germanium

We investigate the interaction effect between oxygen impurities in crystalline germanium on the basis of a quantum rotor model. The dipolar interaction of nearby oxygen impurities engenders non-trivial low-lying excitations, giving rise to anomalous behaviors for oxygen-doped germanium (Ge:O) below a few degrees Kelvin. In particular, it is theoretically predicted that Ge:O samples with oxygen-concentration of 10$^{17-18}$cm$^{-3}$ show (i) power-law specific heats below 0.1 K, and (ii) a peculiar hump in dielectric susceptibilities around 1 K. We present an interpretation for the power-law specific heats, which is based on the picture of local double-well potentials randomly distributed in Ge:O samples.

High-resolution local vibrational mode spectroscopy of the negatively charged oxygen–vacancy complex in germanium

It was recently discovered that in irradiated germanium doped with oxygen a local vibrational mode occurs at 669cm−1 which could be ascribed to the negatively charged oxygen–vacancy (VO−) complex. In the 669cm−1 band and in another band at 731cm−1 due to a different defect, fine structure which indicated the occurrence of the Ge-isotope splitting of the modes could be partly resolved. We report here the results of high-resolution infrared measurements (res.=0.02cm−1) at liquid helium temperatures of the two bands. The oxygen-doped germanium ([Oi]=2.7×1017cm−3) was first irradiated with 2MeV electrons to increase the concentration of VO complexes. A second similar specimen received the same treatment but with subsequent anneals to obtain the 731cm−1 band. The experimentally observed splitting of the 669cm−1 band and the amplitudes of its individual components are accurately predicted using a model of a non-linear symmetric Ge–O–Ge molecule within a Ge crystal lattice, in agreement with the assignment of the mode to the VO− complex. A similar model also seems to apply to the yet unassigned 731cm−1 mode and the conclusion is made that the defect also must include a single O atom bonded to two equivalent Ge atoms. The higher annealing temperature of the unassigned band however indicates that the defect must be more complex than the VO center.

High sensitivity bolometers development

A main drawback of bolometers is their low responsivity resulting in very poor time and/or spatial resolutions. Improvement of detectors responsivity would serve to solve this problem and bolometers could be useful in studies of fast-developing phenomena in plasmas. We are searching for the development of new detectors based on a standard multilayer design and having as sensitive material amorphous germanium. The chosen detector structure is rigid substrate (low conductivity glass)/interdigitated electrodes (Ti-Au defined by photo- lithographic techniques)/resistor layer (amorphous oxygen-doped germanium evaporated by electron beam)/insulation layer (photo-CVD deposited Si3N4)/absorber foil (evaporated + electrolytic gold layers). The detector performance has been compared with pyroelectric detectors and germanium bolometers in the TJ-I tokamak under the same experimental conditions. Also, the design and construction of monolithic miniaturized detector arrays, based on this concept, are in progress.

Bistability of thermal donors in germanium: Assignment of far-infrared and electron-paramagnetic-resonance spectra.

Bistability of different thermal donor (TD) species in oxygen-doped germanium has been observed in far-infrared and EPR spectra. The phenomenon has been used to establish the correspondence of the spectra found in the two experiments, resulting in the identification of the EPR spectrum of the species TD2 to TD5. The spectra reveal ${\mathit{C}}_{2\mathit{v}}$ symmetry for TD2 and TD5 and ${\mathit{C}}_{3\mathit{v}}$ symmetry for TD3 and TD4. The results are explained in terms of neutral double donors with S=1. In addition, the spectrum of singly ionized TD species has been observed with use of compensated samples. The spectrum, which is assigned to a superpositon of TD3 and TD4, reveals trigonal symmetry and the data can be fitted assuming S=1/2.

Nature of Oxygen Donors and Radiation Defects in Oxygen-Doped Germanium

The nature of oxygen donors and radiation defects in oxygen-doped germanium were studied through measurements of the infrared absorption spectrum, deep level transient spectroscopy spectrum and carrier concentration. It is revealed that a new donor is not formed in oxygen-doped germanium. An A-center (interstitial oxygen-vacancy pair) forms a complex with a thermal donor in its anneallng stage at 60°C-140°C. The introduction rate of defects by 1.5 MeV electron irradiation was enhanced in thermal-donor-doped samples.

Electron paramagnetic resonance of thermal donors in germanium.

At least two new electron-paramagnetic-resonance (EPR) spectra were observed in oxygendoped germanium. One of them, which is ascribed to thermal donors, reveals ${C}_{2v}$ symmetry in agreement with similar centers in silicon. The results are explained in terms of defects compressing the neighboring lattice along a [110] direction. Another spectrum exhibits probably ${C}_{3v}$ symmetry, but its correspondence with thermal donors remains speculative. The EPR results also show that the effective spin of these centers should be larger than \textonehalf{}.

Thermal Donor Formation and the Effect of Copper Contamination in Oxygen-Doped Germanium

The structure of thermal donors in germanium was studied in oxygen-doped crystal (9×1016 oxygens/cm3). From the reduction of interstitial oxygen concentration after annealing, it was found that a single thermal donor contains three oxygen atoms on average. The diffusion of copper atoms from the surface in oxygen-doped germanium crystal was studied through electron irradiation effects. It was revealed that a copper atom migrates as an interstitial atom and it occupies a substitutional site at about 450°C.

Unified model for the thermal donor energy spectra In silicon and germanium

The hierarchy of thermal donors produced by annealing oxygen-doped germanium at ≈ 425°C is explained by using the perturbation model that has been applied to the thermal donors in silicon, with the only change being a scaling of the physical constants in order to reflect the different crystalline environment.

Excitation spectrum of D 0 and D + states of double donors in oxygen doped germanium

Optical spectra for D0X (excitons bound by neutral donors) are identified in emission and absorption spectra of CdTe/Cd1−xZnx multiple quantum wells that were planar-doped with indium. The effect of quantum confinement on the exciton localization energy is measured as a function of well width. The 1s → 2p+ transition of neutral donors is studied by far-IR magnetoabsorption spectroscopy and shows no evidence for segregation of the dopant atoms.

Spectroscopic observation of double donors in oxygen-doped germanium

Far-ir absorption spectra of thermal oxygen donors in germanium provide evidence for their double-donor nature. This follows from the transformation of the spectrum upon thermal ionization from ${D}^{0}$ to ${D}^{+}$ and from the $2p\ifmmode\pm\else\textpm\fi{}\ensuremath{-}2{p}_{0}$ spacing of ${D}^{+}$. A comparison with theoretical energy values confirms the picture of heliumlike donors. Several donors are found with binding energy in the 16-17.3-meV (${D}^{0}$) and 34-37-meV (${D}^{+}$) ranges.

DLTS and PTIS of new donors in oxygen-doped germanium

Two new DLTS bands I and II have been observed in n-type germanium containing 10 16 cm -3 interstitial oxygen. The signature of II is: E c−E II = 0.031 eV, K = 4 × 10 7 cm −2s −1; level I is about half as deep. In addision, the excitation spectrum of I is recorded by PTIS. It is composed of three hydrogenic donor series Ia, b, c with activation energies 17.25, 17.6 and 18.1 meV. The donors I and II are thought to correspond with the well known thermal oxygen donors. Energies for the latter were determined at 0.017 and 0.04 eV from Hall effect by Fuller and Doleiden. The possibility of double donors is examined, in analogy with the case of oxygen donors in silicon.

Electron Bombardment of Heat-Treated Oxygen-Doped Germanium

Electron Bombardment of Heat-Treated Oxygen-Doped Germanium

Oxygen-Defect Complexes in Neutron-Irradiated Germanium

The recovery processes in neutron-irradiated oxygen-doped germanium have been studied by measuring the growth and decay of infrared absorption bands associated with oxygen-defect complexes. After a neutron irradiation at −50°C, the absorption band assigned to the stable oxygen-vacancy complex is barely detectable but increases upon annealing to 50°C. These results indicate that additional vacancies are produced by the dissociation of the large defect clusters formed by neutron irradiation. Annealing to higher temperatures produces the same bands in the same sequence as those which developed in the spectra of electron irradiated samples upon annealing. Comparisons of the annealing curves of the absorption bands produced by the two types of irradiations, however, reveal a general shift to higher annealing temperatures for the neutron-irradiated samples. These temperature shifts are interpreted as resulting from changes in equilibria of the oxygen-defect complexes caused by the dissociation of clusters into simpler defects. No similar evidence for the presence of clusters above annealing temperatures of 350°C was observed.

Two New Paramagnetic Centers in Irradiated, Oxygen-Doped Germanium

Two New Paramagnetic Centers in Irradiated, Oxygen-Doped Germanium

Electron Paramagnetic Resonance in Irradiated Oxygen-Doped Germanium

Electron paramagnetic resonance studies have been performed in oxygen-doped germanium. Room-temperature irradiation with 2-MeV electrons produces a paramagnetic center of S = ½ whose g tensor has (011), (01̄1), (100) principal axes. The g shifts are consistent with this being the germanium A center, i.e., an oxygen-vacancy complex. Infrared absorption bands at 766, 807, and 847 cm−1 may be associated with this defect.

Electrical and optical investigations of the donor formation in oxygen-doped germanium

The absorption of atomically dispersed oxygen in germanium is discussed and two corresponding absorption bands at 7.9μ (1260 cm −1) and 11.7μ (855cm −1) reported. During annealing at T < 500°C, donor states, D, in concentrations up to 10 17 cm −3 form and a simultaneous loss of dissolved oxygen, Δ N, can be observed. The ratio Δ N/ D is found to be 4 after prolonged heating at 470°C and 350°C, indicating that four oxygen atoms join together to form one donor. A new absorption band corresponding to this donor was found at 12.7μ (786 cm −1.)

Kinetics of donor reactions in oxygen-doped germanium

The kinetics of the reactions leading to the formation of donors in oxygen-doped Ge have been investigated by means of resistivity and Hall effect measurements. Reaction rates have been measured for oxygen concentrations ranging from 0.9 to 12.0×10 17 cm −3 oxygen over the temperature range, 300–550°C. The apparent order of reaction is shown to increase with both temperature and oxygen concentration and to approach a limiting value of four. Although the reactions are complex in the kinetic sense, and consist of series as well as parallel reactions, both the forward and reverse reactions are shown to follow approximate first order rates over limited temperature ranges. The reaction of oxygen in the presence of copper is shown to result in the production of a neutral or paired conformation of the Cu and O atoms. The activation energy for donor generation in Cu-doped Ge is 47 kcal (2.0 eV) in excellent agreement with the activation energy for oxygen diffusion.

A differential absorption spectrometer for the infra-red

A method is described of measuring the small change in absorption coefficient due to the introduction of an impurity into a semiconductor. A chopped monochromatic beam of infra-red radiation is passed alternately through the doped and undoped halves of a compound specimen, and the resulting modulation measured. A change of less than 10-3 cm-1 can be observed in the presence of substantial background absorption. The method is illustrated by a measurement on oxygen-doped germanium.