Acceptors In Germanium Research Articles

Electric field ionization of gallium acceptors in germanium induced by single-cycle terahertz pulses

The electric field ionization of gallium acceptors in germanium was studied by using terahertz time-domain spectroscopy after single-cycle terahertz pulse excitation. As the peak electric field of the excitation pulse increases, the distinct absorptions due to acceptor transitions centered at 2.0 and 2.2 THz decrease, and simultaneously, absorption emerges in the lower frequency region. These behaviors clearly show that the terahertz pulse ionizes neutral acceptors. The electric field dependence of the released hole density is well reproduced by a model assuming direct field-assisted tunneling of acceptors.

Relaxation of the shallow acceptor center in germanium

Polarized negative muons were used to study relaxation mechanisms of shallow acceptors in germanium. In p-type germanium at low temperatures relaxation of the muon spin was observed, indicating that the muonic atom (gallium-like acceptor center) formed via capture of the negative muon by a host atom is in the paramagnetic state and its magnetic moment is relaxing. The relaxation rate of the muon spin was found to depend on temperature and on concentration of gallium impurity. We conclude that to the relaxation of the magnetic moment of the Ga acceptor in Ge there contribute both scattering of phonons and quadrupole interaction between the acceptors. We estimate, for the first time, the hyperfine interaction constant for the gallium acceptor in germanium as 0.11 MHz.

Electronic properties of iron and cobalt impurity centres in germanium

The electronic properties of the 3d transition metal impurities iron and cobalt in crystalline germanium have been investigated by means of deep level transient spectroscopy. The metals were implanted at 90 keV and diffused deeper into the bulk during a thermal anneal at 500 °C, which yielded spectra specific for the implanted metal. It was found that Fe introduces one deep electron trap and one deep hole trap. For Co, one deep electron trap and two hole traps were observed. The capture cross-section of both electron and hole traps has been directly measured. The results are in agreement with the metals being present on substitutional lattice sites forming multiple acceptor states. It is concluded that Fe and Co are double acceptors in germanium, with Co having an additional donor level close to the valence band.

Modification to the central-cell correction of germanium acceptors

In this paper, we report a correction to the model potential of the Ga acceptor in germanium, evidenced by high-magnetic-field photoconductivity measurements. We found that under high magnetic fields the chemical shift of the binding energy of Ga acceptors vanishes, contrary to the results given by the generally accepted theory. To fit our data, we found that the central-cell correction should contain a repulsive part (i.e., it must be bipolar), in contrast to the purely attractive screened point-charge potential widely used in the literature.

Optical and magnetic properties of Mn+-implanted GaAs

Neutron-transmutation-doped GaAs samples were prepared by irradiating the middle-level neutrons into the semi-insulating GaAs grown by a liquid encapsulated Czochralski method and subsequently implanted with Mn+. The characteristics of the Mn+-implanted neutron-transmutation-doped GaAs (namely, the implantation of Mn+ subsequent to neutron-transmutation-doping) were investigated by various measurements. The result of the energy dispersive x-ray peak displayed an injected Mn concentration of 9.65%. The photoluminescence peaks related to carbon and germanium acceptors were resolved, and the peaks related to Mn due to a neutral Mn acceptor were evidently observed. It is found that the proper activation for the neutral Mn acceptor starts from a relatively low annealing temperature of 600°C for 15min. The atomic force microscopy and magnetic forcemicroscopy images showed that magnetic clusters were well formed. The ferromagnetic hysteresis loop measured at 10K was observed, and the temperature-dependent magnetization revealed that the two different phases exist at 135 and 360K. The Curie temperature (Tc∼360K) is caused by MnAs, which agrees with the clusters of the magnetic force microscopy image.

Coulomb related Landau spectra of axial and double acceptors in germanium

The absorption spectra of Landau features associated with Coulomb states of the axial single-hole acceptors ZnH and CuX and the double-hole acceptor, neutral zinc (Zn0), all in Ge, have been observed. The magnetic field B was oriented along 〈111〉 for ZnH and Zn0 and 〈100〉 for CuX. The main Landau features for these are the same as those of the group III acceptors in Ge. The differences are in the fine structure of the Landau lines. Under B, the ground state of B consists of four Zeeman levels, those of ZnH and CuX are doublets while that of Zn0 is singlet. Coulomb-related Landau state are the same for B, ZnH and CuX but not so for Zn0. The excited states of the latter are revealed to be compound two-hole states.

Spectroscopy of the hole population in bound excited acceptor states during recombination in p-type Ge

We have designed a new type of far-infrared (FIR) pump-and-probe experiment to investigate the hole population in bound excited states of hydrogenic acceptors in germanium during hole capture. In our experiment holes are pumped from the acceptor ground state to the valence band by a blackbody emitter. The bound excited states of the acceptor are probed for captured holes by FIR-laser radiation with photon energies large enough to re-excite holes to the valence band. The re-excited holes are detected by measuring the FIR-laser induced photoconductive response of the Ge samples. We have been able to prove for the first time in a direct measurement that a fraction of the recombining holes are indeed captured into excited bound acceptor states.

Zeeman and Piezo-Zeeman spectroscopy of acceptors in germanium

A brief review is given of the Zeeman and piezo-Zeeman results for the spectra of shallow acceptors in germanium. Also included are new data for the Zeeman spectra of thallium with B∥〈100〉 and boron, gallium and thallium under B∥〈111〉. The observations are for field strengths up to 7T and the results compared with recent calculations of these effects. New data for neutral zinc in germanium with B∥〈100〉 is also indicated.

High-field Zeeman effect of shallow acceptors in germanium.

Zeeman absorption spectra have been obtained for B and Ga in Ge for B\ensuremath{\parallel}〈100〉 in the Voigt configuration with plane-polarized radiation. All twelve allowed transitions were observed for both the G and D lines. The corresponding excited states of these two lines for both impurities behave identically; two recent theoretical results are in good agreement. The measurements are a sensitive probe of the ground states; there are differences between the behavior of these for the two acceptors.

Zeeman and Piezo-Zeeman Spectroscopy of Zinc and Copper Acceptors in Germanium

Zeeman and Piezo-Zeeman Spectroscopy of Zinc and Copper Acceptors in Germanium

Zeeman and Landau Spectroscopy of Group III Acceptors in Germanium

Zeeman and Landau Spectroscopy of Group III Acceptors in Germanium

High-Purity Germanium Technology for Gamma-Ray and X-Ray Spectroscopy

ABSTRACTHigh-purity germanium (HPGe) for gamma-ray spectroscopy is a mature technology that continues to evolve. Detector size is continually increasing, allowing efficient detection of higher energy gamma rays and improving the count rate and minimum detectable activity for lower energy gamma rays. For low-energy X rays, entrance window thicknesses have been reduced to where they are comparable to those in Si(Li) detectors. While some limits to HPGe technology are set by intrinsic properties, the frontiers have historically been determined by the level of control over extrinsic properties. The point defects responsible for hole trapping are considered in terms of the “standard level” model for hole capture. This model originates in the observation that the magnitude and temperature dependence of the cross section for hole capture at many acceptors in germanium is exactly that obtained if all incident s-wave holes were captured. That is, the capture rate is apparently limited by the arrival rate of holes that can make an angular-momentum-conserving transition to a s ground state. This model can also be generalized to other materials, where it may serve as an upper limit for direct capture into the ground state for either electrons or holes. The capture cross section for standard levels σS.L. is given bywhere g is the degeneracy of the ground state of the center after capture, divided by the degeneracy before capture. Mc is the number of equivalent extrema in the band structure for the carrier being captured, mo is the electronic mass, m* is the effective mass, and T is the temperature in degrees Kelvin.

Theoretical model for hole capture at acceptors in germanium.

Experimental results on hole capture in germanium are reviewed; good agreement is found not only among various investigators for the same center, but among different acceptors regardless of charge state. For these acceptors, the capture cross section depends only on temperature (\ensuremath{\propto}${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$). Collision theory is used to transform temperature dependence to wave-vector dependence, \ensuremath{\sigma}=\ensuremath{\pi}/${\mathit{k}}^{2}$. This is equal to the s-wave component of the incident plane wave decomposed into polar coordinates by partial wave analysis. It is proposed that direct capture into the ground state exceeds cascade capture for T>3 K for capture at singly ionized acceptors and for T>11 K for capture at doubly ionized.

Acceptor excitation spectra in germanium in a uniform magnetic field.

The irreducible-spherical-tensor form of the effective-mass Hamiltonians for acceptors in cubic semiconductors in a homogeneous magnetic field parallel to a 〈001〉 direction or to a 〈111〉 direction has been derived. The Hamiltonians take into account the full structure of the ${\mathrm{\ensuremath{\Gamma}}}_{8}^{+}$\ensuremath{\bigoplus}${\mathrm{\ensuremath{\Gamma}}}_{7}^{+}$ valence-band edge, and they contain a point-charge potential with spherically symmetric q-dependent dielectric screening. The eigenstates have been calculated variationally for the case of acceptors in germanium. For both orientations, the computations have been performed as a function of the magnetic-field strength, in the range from 0 to 5 T. The binding energies of the first 36 odd-parity excited states and of the first 4 even-parity states (which form the ground-state multiplet) are obtained. The oscillator strengths of the electric-dipole transitions from the ground-state sublevels to the excited states are also calculated, and the results are used to simulate theoretical acceptor excitation spectra, which are in excellent agreement with the available experimental far-infrared-absorption and photothermal-ionization-spectroscopy spectra. For the ground state and for the final states of G and D lines, theoretical g factors are obtained which are very close to the experimental ones. In particular, the values ${\mathrm{g}}_{1}^{\ensuremath{'}}$=-0.45 and ${\mathrm{g}}_{2}^{\ensuremath{'}}$=+0.22 are computed for the acceptor ground state. It is found also that for the ground state and for the excited ${\mathrm{\ensuremath{\Gamma}}}_{8}$(T${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{d}}$) final states of the G, D, and B spectral lines, the ratio r=${\mathrm{g}}_{2}$/${4\mathrm{g}}_{1}$ is always very close to the special values -541, which corresponds to a linear Zeeman splitting of the ${\mathrm{\ensuremath{\Gamma}}}_{8}$ states into a degenerate doublet for B\ensuremath{\Vert}〈111〉 and into an equally spaced quartet for B\ensuremath{\Vert}〈001〉.

Nonlinear far-infrared photoionization of shallow acceptors in Germanium

The optical saturation of nonresonant photoionization of shallow acceptors in p-Ge was investigated at low temperatures using a high-power FIR molecular laser. The experimental results were analyzed in terms of a rate equation model. Very small saturation intensities in the order of 10 mW/cm −2 and long free hole recombination times in the range of 10 μs were observed.

Coulomb-interaction effects on the excited states of double acceptors in germanium and silicon.

Infrared spectra of double-acceptor impurities in Si and Ge show small splittings. This fine structure cannot be explained by a split ground-state manifold only. An analysis of the Coulomb hole-hole splitting of the excited states, an estimate of the parameters involved, and dipole transition probabilities between the various ground-state symmetries and the excited states are presented. The spacing of accessible states may be large enough to be experimentally resolved.

Absorption cross sections in the far-IR spectrum of elemental shallow donors and acceptors in germanium

The integrated absorption cross section of the main lines in the far-IR spectrum of the group-V donors Sb, P, and As and the group-III acceptors B, Al, Ga, and In in germanium has been determined for impurity densities in the range of 1012–1014 atoms/cm3. The cross sections and consequently the experimental oscillator strengths for transitions from the ground state, decrease gradually with increasing ground-state energy. In compensated samples the electric field due to the ionized impurities strongly affects the relative line strengths; neutralizing by band-gap illumination allows one to minimize the effect of the electric field and to observe simultaneously the majority and minority impurities, allowing one to calculate the compensation.

Oscillator strengths of shallow impurity spectra in germanium and silicon.

The oscillator strength of dipole transitions of donors in silicon and germanium and of acceptors in germanium is calculated in the effective-mass approximation. A point-charge potential including variable screening allows one to adjust the ground state to the range of experimental binding energies and to explore the effect of the central-cell correction on the oscillator strength. Calculated f values are listed up to 7${p}_{\ifmmode\pm\else\textpm\fi{}}$ for the different group-V donors and up to 9${\ensuremath{\Gamma}}_{8}^{\mathrm{\ensuremath{-}}}$ for the different group-III acceptors. A comparison with recent experimental oscillator strengths is made. The results are in fair agreement with the experimental data concerning the structure of the spectra and the chemical trend of the absorption cross section. The assignment of the C line and the A multiplet in the spectrum of acceptors in germanium is substantiated.

Piezospectroscopy of the ground and excited states of zinc double acceptors in germanium.

Piezospectroscopic midinfrared absorption studies of the neutral double acceptor Zn in Ge have been undertaken to determine the ordering and symmetries of the ground and excited states. The results are shown to be in excellent agreement with existing theoretical models of the double acceptor states, provided that the lowest-energy component of the split ground state has ${\ensuremath{\Gamma}}_{1}$ symmetry at zero stress. The experimentally determined deformation-potential constants for Zn agree well with those of the single acceptor Ga in Ge.

Absorption strengths in the far-IR spectrum of shallow donors and acceptors in germanium

The far-IR absorption spectrum of elemental shallow donors and acceptors in germanium has been studied as a function of the dopant concentration (10 12–10 14 at/cm 3). The integrated absorption cross section and the oscillator strength have been determined for the main transitions from the ground state. The oscillator strengths decrease gradually in the sequences Sb, P, As and B, Al, Ga, In, i.e. with the ground state becoming gradually deeper. Absorption spectra of compensated samples are also presented, illustrating the effect of band-gap illumination on the relative line strengths of the compensated as well as the compensating impurities.