Acceptors In Germanium Research Articles

Photoluminescence and infrared absorption studies of double acceptors in germanium

Recent studies of double acceptors using near-, mid-, and far-infrared spectroscopy are reviewed, and new results are presented in each of these areas. Bound excitons and bound multiexciton complexes associated with these centers are investigated using photoluminescence and near-infrared absorption spectroscopy, and also measurements of the far-infrared absorption due to transitions between bound exciton states. Many bound exciton excited states are observed, and are well explained by a pseudo-donor model. The earlier predictions that the double acceptor ground states were split by hole-hole coupling, which were based upon near-infrared results and the temperature dependence of the mid-infrared neutral acceptor absorption, are verified by detailed uniaxial stress measurements of Ge:Be and Ge:Zn.

IR-spectroscopy of impurity complexes in germanium

A large number of impurity complexes which create shallow levels, have been discovered in pure and doped germanium single crystals. The structure and the composition of several of these complexes have been determined using far-infrared spectroscopy and information about crystal growth related residual impurities. In general, the complexes consist of a heavy substitutional impurity binding one or more light. interstitial impurities such as hydrogen or lithium. The coupling between the tunneling motion of the interstitial component and the electronic structure of the complexes can create complicated ground state manifolds which contain stress insensitive components. Spectroscopy further shows that multivariate centers, such as double or triple acceptors, become hydrogenic complexes upon binding one or two hydrogen atoms, respectively. Neutralization of shallow acceptors in silicon, which has recently attracted attention, appears to be the same phenomenon as the valency reduction of multiple acceptors in germanium.

Shallow Positive Acceptor in Germanium Doped with Deep Zinc Impurity

An A + center, a positively charged acceptor, is observed below 2.8 K in far-infrared photoconductivity measurement on Zn doped Ge under exposure to the room temperature black body radiation. The hole binding energy of the A + center is estimated as 1.9 meV from the low energy threshold of photoconductivity. Low temperature photoconductivity for the sample with low impurity concentration ( N A =1.2×10 14 cm -3 ) is due to isolated A + centers. The photoconductivity peak shifts to higher energy with increase of impurity concentration. The shifted peak corresponds to the transition from an A + related complex to the valence band. Magneto-oscillations of photoconductivity due to isolated A + centers are first observed. The peak photon energies converge to 1.9 meV in the zero field limit.

Donor-Like Zeeman Absorption of Exciton Bound to Zinc Acceptor in Germanium

Internal transition of an electron in an exciton bound to a zinc acceptor in germanium has been observed by time resolved far-infrared magneto-absorption. A donor like model of an acceptor bound exciton explains most of the obtained absorption spectra except for the central cell correction of 1s like state and the doublet structure. The bound exciton lines have been assigned to be 1s to 2p ± like transition of electrons in the two kinds of valleys. The lifetime of the bound exciton and the capture cross section of an exciton by a neutral acceptor are estimated to be 0.75 µs and 1.2×10 -15 cm 2 , respectively, by time-resolved absorption measurement.

A shallow hydrogen-zinc acceptor in germanium

A scalable and portable code named Atomsviewer has been developed to interactively visualize a large atomistic dataset consisting of up to a billion atoms. The code uses a hierarchical view frustum-culling algorithm based on the octree data structure to efficiently remove atoms outside of the user's field-of-view. Probabilistic and depth-based occlusion-culling algorithms then select atoms, which have a high probability of being visible. Finally a multiresolution algorithm is used to render the selected subset of visible atoms at varying levels of detail. Atomsviewer is written in C++ and OpenGL, and it has been tested on a number of architectures including Windows, Macintosh, and SGI. Atomsviewer has been used to visualize tens of millions of atoms on a standard desktop computer and, in its parallel version, up to a billion atoms.Title of program: AtomsviewerCatalogue identifier: ADUMProgram summary URL: http://cpc.cs.qub.ac.uk/summaries/ADUMProgram obtainable from: CPC Program Library, Queen's University of Belfast, N. IrelandComputer for which the program is designed and others on which it has been tested: 2.4 GHz Pentium 4/Xeon processor, professional graphics card; Apple G4 (867 MHz)/G5, professional graphics cardOperating systems under which the program has been tested: Windows 2000/XP, Mac OS 10.2/10.3, SGI IRIX 6.5Programming languages used: C++, C and OpenGLMemory required to execute with typical data: 1 gigabyte of RAMHigh speed storage required: 60 gigabytesNo. of lines in the distributed program including test data, etc.: 550 241No. of bytes in the distributed program including test data, etc.: 6 258 245Number of bits in a word: ArbitraryNumber of processors used: 1Has the code been vectorized or parallelized: NoDistribution format: tar gzip fileNature of physical problem: Scientific visualization of atomic systemsMethod of solution: Rendering of atoms using computer graphic techniques, culling algorithms for data minimization, and levels-of-detail for minimal renderingRestrictions on the complexity of the problem: NoneTypical running time: The program is interactive in its executionUnusual features of the program: None

Spectroscopy of positively charged multiple acceptors

Experimental results from far-infrared photoconductive spectroscopy of positively charged multiple acceptors in germanium, i.e. those with three or four bound holes, will be explained in terms of a theoretical model. The fundamental concept is that of the pseudo-atom, making use of pseudo-electrons with spin quantum number s= 3 2 . This allows up to four-fold occupation of the (1s)-like ground state spatial envelope function. Optical pumping and stress experiments for the case of Ge:Be demonstrate the qualitative features of the model. Satisfactory agreement is obtained for the energy levels of the positively charged double acceptors using an effective Rydberg approach. A new physical system which does not exist in isolated atoms has been studied for the first time, for which the binding of a third (or fourth) particle for Z=2 (or 3) becomes possible.

Far-infrared study of exciton binding by zinc acceptor in germanium

The excited state spectra of the Group III acceptors in Si and Ge are calculated using a semi-empirical short range potential which is adjusted in each case to give the observed ground state binding energy. The model is in good agreement with the previously published data on the chemical shifts of the even-parity s-like excited states, as well as with our new measurements of the energies of these states for the deep acceptor In in Si. For odd-parity states, the model shows that the chemical shifts are negligibly small, in agreement with experiment.

Binding energy of positively charged acceptors in germanium - a Green's function Monte Carlo calculation

Recent spectroscopic measurements in Ge reveal positively charged acceptors consisting of an extra hole bound to a double or triple acceptor. Using the effective-mass approximation and a pseudo-atom model which permits up to four holes in the 1 s ground state, we calculate the binding energies of these positively charged acceptors by the Green's function Monte Carlo method. For double acceptors the results of the pseudo-atom model compare favorably with experiments, whereas for triple acceptors the differences are significant.

Quenched-in deep acceptors in germanium

Quenching from temperatures in the range 500-750 degrees C of high-purity p-type germanium induces a deep hole trapping state at Ev+0.23 eV. This level is also seen in gamma -irradiated samples from the same material, and is tentatively identified as an oxygen-vacancy complex. Quenching from 700 degrees C of copper-doped germanium induces the normal Cu-, Cu2- and Cu-H levels, as well as a deep hole trap at Ev+0.41 eV. Field-assisted emission measurements are consistent with this level being due to triply ionised copper.

Radiative recombination of excitons bound to beryllium double acceptors in germanium

Beryllium-doped germanium single crystals exhibit at low temperatures novel sharp-line exciton spectra at photon energies essentially lower than those familiar from studies of excitons localised at shallow donors or acceptors. The spectra consist of two strong principal lines and two weak lines. This structure is observed in no-phonon luminescence as well as when momentum-conserving TA, LA, LO and TO phonons participate. The authors study the lines as a function of excitation power, temperature, doping level and under uniaxial stress along (001). The results strongly suggest that three of the lines are due to multiple bound excitons localised at neutral beryllium acceptors, and one line originates in an excited state of the (single) bound exciton.

Near-band-edge luminescence in heavily doped gallium arsenide

The near-band-edge photoluminescence at 80K of heavily tellurium-doped degenerate liquid-phase-epitaxial gallium arsenide layers (n=2*1018 cm-3) is investigated. The layers are compensated with shallow germanium acceptors (the values of the degree of compensation are K=0.05-0.7) during the epitaxial growth process. It is established that even at very low concentration of shallow acceptors (K=0.05), the so-called mobile band appears in the near-band-edge luminescence spectra. The peak energy of this band shifts to lower energies, the non-equilibrium carrier generation rate decreases and saturates at values E0T, which are considerably lower than those calculated for the filling of the valence band density-of-states tails. It is assumed that this discrepancy between theory and experiment is due to the uncertain knowledge of the pre-exponential term in the expression for the density-of-states tails and to the formation of some deeper fluctuation states when there is a considerable difference between the concentrations of shallow donors and acceptors. It is assumed that in this case some of these states act as associates.

Graphic representation of the ground state wavefunctions of the shallow acceptor in germanium

Surfaces representing the loci of points of which psi *(r) psi (r) = psi *(O) psi (O)exp(-2r/a), where psi (r) are the wavefunctions of the components of the acceptor ground state in Ge, and a is the average Bohr radius of the acceptor ground state, are presented for several values of the uniaxial stress applied in the (001) orientation. The value of r/a was selected to be equal to 5 and consequently the surfaces give the location in space of points where the tunnelling probability to the point at the origin is equal and sufficiently small to represent the phonon-assisted hopping transitions. The authors demonstrate that the surface of constant tunnelling probability are deformed by the stress in a manner consistent with the assumptions involved in the model of piezoresistance of hopping conduction in p-type germanium formerly proposed by Chroboczek et al. (1981).

Double acceptor bound exciton in Ge

Six photoluminescence lines from excitons bound to neutral zinc acceptors in germanium are observed. The six lines consist of three replicas associated with zero, TA and LA phonons. Two lines from each replica correspond to the transitions from the split ground states of the bound exciton complex. The binding energy of the bound exciton is found to be 3.2 meV.

Far-infrared photoconductivity spectra of quenched-in acceptors in germanium: Chemical effects on the SA 1 acceptors

Far-I.R. photoconductivity reveals the existence of at least four different kinds of SA 1 acceptors in germanium quenched from T > 800 each acceptor producing two or more hydrogenic excitation line series. One acceptor is correlated with the presence of Ni, another one with strong Cu contamination. The two other acceptors, which we call ‘normal’ and ‘shifted’, occur in samples quenched without intentional doping. The experiments indicate that the SA 1 acceptors in germanium are impurity complexes involving fast diffusing elements. We discuss a model according to which the SA 1 acceptors would be interstitial-substitutional pairs of transition metals. In particular, we propose to assign the ‘normal’ SA 1 acceptors to Fe I-Cu S pairs.

Photoluminescence studies of the amphoteric behavior of carbon and germanium in GaAs

Recombination radiation due to an exciton bound to neutral donors and neutral acceptors in high purity vapor phase epitaxial GaAs is investigated using high resolution photoluminescence spectroscopy at liquid helium temperatures. It is found that those samples which show the presence of germanium acceptors also exhibit a strong residual donor referred to in the literature as X 3. Samples containing carbon acceptors however, do not show the presence of X 3. In the past X 3 has been identified by some groups as due to carbon on the gallium site. The work presented here sugests that the X 3 donor is associated with germanium. This identification of the X 3 donor is in agreement with a recent assignment based on the far infrared study of neutron transmuted GaAs.

Far-infrared photoconductivity spectra of quenched-in acceptors in Germanium: absorption and photo-thermal ionization lines of two new shallow acceptors

The far-infrared photoconductivity spectra of germanium crystals quenched from T>800C show three new sets of hydrogenic acceptor excitation lines, in addition to the 8.69 meV (SA ' 1) and 9.48 meV (SA “ 1) acceptor series reported earlier. The corresponding ground state binding energies are 13.89 meV (SA ' 2), 14.42 meV (SA “ 2), and 17.89 meV (SA 3). We tentatively attribute both the SA ' 2 and the SA “ 2 line series to one acceptor complex with a multiple ground state. At high acceptor concentrations the excitation lines are seen as sharp minima superimposed on a continuous photoconductivity background. With decreasing acceptor concentration, or increasing temperature, they gradually evolve into the familiar positive-going photo-thermal ionization peaks. This is explained by the different concentration and temperature dependence of the cross-sections for the absorption and photo-thermal ionization mechanisms.

Jj-Type of coupling in the scheme to construct the energy levels of a shallow acceptor in semiconductors

There is examined the classification of shallow acceptor energy levels by jj-type coupling, whose wave functions are converted by sets of irreducible representations by one of the subgroupsD 4h /′ ,D 3d /′ ,D 2h /′ of the group o h /′ . The representations Γ 5 + of the groupD 4h /′ are obtained in the one-function approximation by using a variational method, and systems of eight radical second-order differential equations are solved for two functions Γ 5 + in the case of a shallow acceptor in germanium by the method of orthogonal differential factorization. A comparison is made of the two low levels found and their radial functions for each of the functions Γ 5 + with the computed levels and the functions of LS-type coupling.

A PTIS study of quenched-in acceptors in germanium

The shallow acceptors produced in germanium crystals by quenching from 820–925 C have been studied for the first time using Photo-Thermal Ionization Spectroscopy (PTIS). We have found two acceptor-continua, which correspond to the E v + 8.4 meV and E v + 12 meV levels observed in earlier Hall measurements. With the lower energy continuum there are associated two previously unobserved hydrogenic acceptors. They are shallower than any known acceptor in germanium: their ionization energies are 8.69 ±0.01 meV and 9.48 ±0.01 meV. We attribute the acceptors to two different defects because of differences in their creation and annealing behaviour. No discrete lines were found to be associated with the higher energy continuum. We estimate the acceptor ionization energy to be about 14 meV. Finally, we have observed a number of as yet unexplained negative lines superimposed on both acceptor-continua.

Ionization energy of quenched-in shallow acceptors in germanium

Ionization energy of quenched-in shallow acceptors in germanium

Spectral diffusion in acceptor-hole system in germanium at low temperature: ultrasonic hole-burning experiment

The interaction among lightly doped acceptors in germanium is observed by an ultrasonic hole-burning experiment. The observed width of the burned hole is ascribed to spectral diffusion due to the exchange and quadrupole interactions of nearest-neighbour acceptor pairs. In a strong magnetic field the exchange interaction is suppressed and hence the width is due to the quadrupole interaction and the single-phonon relaxation process (2T1)-1.