Second-harmonic interferometric spectroscopy of buried interfaces of column IV semiconductors

The technique of combined optical second-harmonic (SH) intensity and phase spectroscopy, which is the spectroscopic modification of SH phase measurements, is proposed to study the nonlinear optical response of semiconductor interfaces with spectrally close resonant contributions. The spectral dependences of SH intensity and phase from oxidised Si (111) and Ge (111) surfaces are studied in the range of 3.5- to 5-eV SH photon energy. The resonant behaviour of combined SH spectra is associated with a superposition of contributions from direct interband transitions at several critical points of Si and Ge band structures. Received: 16 October 2001 / Revised version: 16 April 2002 / Published online: 6 June 2002     

Spectroscopy and imaging of metal-organic interfaces using BEEM

Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electron emission microscopy (BEEM) enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrier injection is observed for both Ag-PPP (poly-paraphenylene) and Ag-MEHPPV (poly-2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) interfaces. BEEM current images are found to correlate only marginally with the surface topography of the Ag film.     

THz spectroscopy of diluted magnetic semiconductors and semiconductor quantum structures in ultrahigh magnetic fields

Magneto-absorption spectra in ferromagnetic semiconductor In_1−xMn_xAs films and self-organized PbSe/PbEuTe quantum dot superlattices have been studied in the terahertz range at very high magnetic fields up to 500 T. Both heavy hole (HH) and light hole (LH) cyclotron resonance (CR) have been observed in bulk In_1−xMn_xAs thin films with different Mn concentrations. The detailed Landau level calculation in terms of the effective mass approximation well explained the CR peak positions, line shapes and the dependence of the circular polarization of the incident light on the CR spectra. In InMnAs/GaSb heterostructures that have higher ferromagnetic transition temperature (T_c) than the bulk samples, the observed HH and LH cyclotron masses are larger than that in the bulk thin films. We found that the CR peak position and its line shape suddenly change in the vicinity of the ferromagnetic transition temperature, suggesting the change in the electronic structure due to the ferromagnetic transition. Electron CR in PbSe/PbEuTe quantum dots has been observed and it was found that the effective mass of the electrons is considerably modified by the quantum confinement potential and the lattice strain around the dots. A large wavelength dependence of the absorption intensity was observed due to the interference effect of the radiation inside the sample.     

Submillimetre spectroscopy of semiconductors

The active areas of semiconductor investigation by means of submillimetre spectroscopy are reviewed. Bulk cyclotron resonance in very high magnetic fields is now being applied not only to classic materials such as InSb but to the newer materials such as GaP and even to the highly conducting materials such as PbTe and HgSe. The quasi-two-dimensional gas formed by the carriers present in the semiconductor-insulator interface in MIS or MOS capacitor structures can also be fruitfully investigated by cyclotron resonance methods. The introduction of the photoconductive method by means of which the specimen becomes its own detector has greatly increased the sensitivity of the study of shallow impurity states. Photoionization studies on silicon have proved not only productive but may also be the basis for a new fast sensitive detector. Time resolved studies of the excitons formed in semiconductors have revealed the existence of new quasi-particles.     

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 A₁ 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.     

Dissipative structures in laminar semiconductors

A laminar electron semiconductor consisting of two branches is considered: passive (with a single-valued homogeneous current-voltage characteristic (CVC), and active (with an S-shaped overheated CVC). The self-consistent nonlinear problem is solved about seeking the dissipative structures (DS), the electron temperature strata and current filaments, that occur in the sandwich under consideration. The system bifurcation characteristics are found. The appearance of DS on the CVC of each of the branches and the specimen as a whole is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 63–70, October, 1985.The author is grateful to F. G. Bass, Yu. G. Gurevich, and V. S. Bochkov for discussing the results of the research.     

Nanoscale imaging of buried structures with elemental specificity using resonant x-ray diffraction microscopy

We report the first demonstration of resonant x-ray diffraction microscopy for element specific imaging of buried structures with a pixel resolution of approximately 15 nm by exploiting the abrupt change in the scattering cross section near electronic resonances. We performed nondestructive and quantitative imaging of buried Bi structures inside a Si crystal by directly phasing coherent x-ray diffraction patterns acquired below and above the Bi M5 edge. We anticipate that resonant x-ray diffraction microscopy will be applied to element and chemical state specific imaging of a broad range of systems including magnetic materials, semiconductors, organic materials, biominerals, and biological specimens.     

Point-contact spectroscopy of electron-phonon interaction in semiconductors

A kinetic theory of current-voltage characteristic of semiconductor-semiconductor and metal-semiconductor point-contact junction is presented showing (a) smooth dependence of differential resistance over the voltage R = R(V), and (b) structure in dR/dV due to phonon emission at energies ?ω < eV.     

Time-resolved IR spectroscopy of quantum-optics in semiconductors

Time-resolved IR spectroscopy is a powerful non-destructive technique for probing electron dynamics and plasmonics in semiconductors. We present recent experiments in which intense IR laser pulses are used to induce “quantum-optical” phenomena, including gain without population inversion and slow light, in semiconductor nanostructures. The potential advantages of IR Synchrotron radiation to probe these systems are discussed.     

Picosecond infrared spectroscopy of molecules and semiconductors

Novel techniques for infrared spectroscopy on the picosecond time scale are presented. Ultrafast unimolecular reactions in organic molecules are studied by vibrational spectroscopy. The nonlinear infrared absorption of hot carriers in semiconductors is investigated.     

Optical spectroscopy of single impurity centers in semiconductors

Using optical spectroscopy with diffraction limited spatial resolution, the possibility of measuring the luminescence from single impurity centers in a semiconductor is demonstrated. Selectively studying individual centers that are formed by two neighboring nitrogen atoms in GaAs makes it possible to unveil their otherwise concealed polarization anisotropy, analyze their selection rules, identify their particular configuration, map their spatial distribution, and demonstrate the presence of a diversity of local environments. Circumventing the limitation imposed by ensemble averaging and the ability to discriminate the individual electronic responses from discrete emitters provides an unprecedented perspective on the nanoscience of impurities.     

Electrical and optical defect spectroscopy of compound semiconductors

Some aspects of recent developments in defect characterization of II–VI, III–V and IV–IV compounds are reviewed. It is shown that in addition to junction space charge techniques high-resolution spectroscopy measurements have provided complementary methods for a more detailed analysis of defects. The application of junction space charge techniques to the study of defects in Si/Ge low-dimensional structures is discussed.     

Electronic structures of impurities and point defects in semiconductors

A brief history of the impurity theories in semiconductors is provided. A bound exciton model is proposed for both donor-and acceptor-like impurities and point defects, which offers a unified understanding for "shallow" and "deep"impurities and point defects. The underlying physics of computational results using different density-functional theorybased approaches are discussed and interpreted in the framework of the bound exciton model.     

Point-contact spectroscopy of electron relaxation mechanisms in semiconductors

The I–V characteristic of a tiny semiconducting channel connecting bulk electrodes is shown to have singularities arising due to phonon emission by hot electrons at energies eV = n?ω₀, where ω₀ is the optical phonon frequency and n = 1, 2, 3,…. The nonlinear part of the I–V curve provides direct information concerning the energy dependence of the elastic-scattering time of charge carriers.     

Femtosecond infrared spectroscopy of semiconductors and semiconductor nanostructures

Infrared spectroscopy on ultrafast time scales represents a powerful technique to investigate the nonequilibrium dynamics of elementary excitations in bulk and nanostructured semiconductors. In this article, recent progress in this field is reviewed. After a brief introduction into electronic excitations below the fundamental bandgap and ultrafast processes in semiconductors, infrared pulse generation and the methodology of time-resolved infrared spectroscopy are reviewed. The main part of this paper is devoted to coherent optical polarizations and nonequilibrium excitations of the electronic system in the spectral range below the fundamental band gap. The focus is on the physics of single component plasmas, i.e. electrons or holes. In particular, intraband, inter-valence and intersubband transitions are considered. Processes of phase relaxation, carrier and energy redistribution are analyzed. The potential of ultrafast infrared technology and spectroscopy for future applications is discussed in the final part.     

Photothermal and photoacoustic effects in semiconductors and semiconductor structures

A theory of photothermal and photoacoustic effects is developed, on which the contactless diagnostics of semiconductors and semiconductor structures are based. Photothermal and photoacoustic effects are characterized quantitatively by the variable temperature of the specimen surface being exposed and by its shift. These quantities are computed in this paper for a homogeneous semiconductor and a semiconductor with a p-n junction with electron transfer processes, heat liberation as a result of thermalization and charge carrier recombination and their passage through the potential barrier as well as nonthermal deformation mechanisms due to nonequilibrium carrier interaction with the lattice in terms of the deformation potential and the reverse piezoeffect taken into account. It is shown that the surface temperature and shift (particularly the phase of these responses) carry information about such semiconductor characteristics as the charge carrier lifetime, the surface recombination rate, the deformation potential constants, the depth of p-n junction location, the height of its potential barrier, etc.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 119–131, June, 1987.     

Acoustic imaging of objects buried in soil

In this study, we demonstrate an acoustic system for high-resolution imaging of objects buried in soil. Our goal is to image cultural artifacts in order to assess in a rapid manner the historical significance of a potential construction site. We describe the imaging system and present preliminary images produced from data collected from a soil phantom. A mathematical model and associated computer software are developed in order to simulate the signals acquired by the system. We have built the imaging system, which incorporates a single element source transducer and a receiver array. The source and receiver array are moved together along a linear path to collect data. Using this system, we have obtained B-mode images of several targets by using delay-and-sum beamforming, and we have also applied synthetic aperture theory to this problem.     

Stability of generation-recombination induced dissipative structures in semiconductors

A class of generation-recombination models involving one type of carriers and several impurity levels is studied. Far from equilibrium, impact ionization can lead to bistability of the homogeneous steady state resulting inS-shaped current-voltage characteristics, and to stationary spatial structures. It is shown for large samples that kink-shaped coexistence profiles are stable, while plane depletion or accumulation layers and cylindrical current filaments are unstable under constant voltage conditions, but can be stabilized by a constant current. The unstable mode is calculated analytically for wide and for narrow depletion layers. The critical slowing-down of this mode is established explicity as a function of the external electric field; it occurs when the threshold or holding field of the switching transitions, or the coexistence field of homogeneous phases is approached. It is shown that the hysteresis cycle of the switching transitions can be shortened by sufficiently large localized fluctuations inducing the nucleation of current layers or filaments.