This symposium was established as an interdisciplinary forum to foster lines of communications among three separate research communities. These communities are: 1) the electro-optic community, 2) the photorefractive community, and 3) the photodetector community. In each of these fields microscopic electric fields are generated inside semiconductor materials by illumination.

The use of large optical nonlinearities in hetero n-i-p-i structures to study charge carrier transport is discussed. A short history of hetero n-i-p-i's and a summary of related semiconductor microstructures are also presented.

We present a model for light-induced charge transport in long-period multiple-quantum-well hetero-nipi structures that is based on Kukhtarev's transport model developed for understanding photorefractive grating formation. We give a simple analytical expression for the steady-state space-charge field in a hetero-nipi under illumination as a function of light intensity.

This paper describes methods of operating Quantum Well SEED's, which utilize the dynamic properties of the devices and thereby forego the need for optical bistability. The major advantages of dynamic operation versus the conventional modes utilizing optical bistability are that much less total optical energy is needed per cycle and/or much faster operation is possible due to much shorter switching times. Such dynamic modes of operation do require a more careful rationing of optical energy from external laser sources and require that the devices have much higher output contrast ratios.

Using the recently developed SESHL (for Single Epitaxy Supporting HBT and Low-barrier modulator) process, we present a monolithic single-stage common-emitter amplifier with photodiode input and modulator output (or HBT-SEED), one of the simplest forms of smart pixels.

We have constructed a completely quantum-mechanical theoretical treatment of the modification of the local dielectric function by intense electric fields. It includes the numerical calculation of depletion-region bound-state energies and wavefunctions as well as band wavefunctions. We use this treatment to construct electroreflectance (ER) lineshapes. Our numerical results differ significantly from those of previously proposed ER lineshapes for GaAs with Nd >> 1016. They yield excellent fits to ER data and allow the accurate determination of doping levels and band bendings from ER data.

A new technique for obtaining the electroreflectance (ER) spectrum, Vacuum Electroreflectance (VER), is introduced. In VER, the modulating electric field is applied capacitively using a transparent electrode as one plate and the sample as the other plate, with vacuum in between. In this geometry, cooling or heating is achieved without any sample preparation. Precise positioning is achieved by piezoelectric motors monitored by a HeNe interferometer, and cooling to 80 K by the Joule-Thompson effect. GaAs samples with different Si doping are used to compare VER to Electrolyte ER and Photoreflectance.

The use of a weak modulated illumination with above-bandgap light energy as a probe scanning a Electrolyte/Dielectric/Semiconductor structure under electrochemical potentiostatic polarization gives an opportunity to obtain qualitative cartographies of different electrical parameters characterizing the Semiconductor/Dielectric interface.

GaAs (100) samples with multiple δ-doped layers (N2D=∼2×1013/cm2) were studied by Raman scattering (RS) and spectroscopic ellipsometry (SE). A quasithree- dimensional (3D) plasmon-phonon coupled mode (L+), probed at λ= 514.5 nm, from a 9-layer δ-doped GaAs with layer-spacing of 100 Å, was observed at ∼895 cm−1. At similar frequency, a plasmon mode was also detected from another GaAs sample with the same δ-doping periods but doubled layer-spacing (200 Å). This provides evidence of spatial quantization of the electron distributions in δ-doped GaAs. The equivalent 3D electron concentration, estimated from the Raman plasmon mode, is ∼1.1×1019/cm3. The presence of the 3D plasmon mode from a quasi-two-dimensional (2D) electron gas is possibly contributed by the electrons in the high energy subbands in the V-shaped potential well of the δ-doped GaAs. The pseudodielectric function <ε>= <ε1>+i<ε2> of this δ-doped GaAs sample was measured by spectroscopic ellipsometry (SE), from an unoxidized surface in an ultrahigh vacuum (UHV) chamber, in the range of 1.5 to 5.0 eV. Compared with uniformly doped GaAs, our SE data indicates a reduced broadening of the optical transitions between the E1 and E1+Δ1, energies due to the δ-doping.

Single Be and Si δ-doped layers in GaAs have been investigated by Raman spectroscopy. GaAs/AlxGa1-xAs heterointerfaces, placed 30nm from the dopant spike on either side, facilitate optical measurements by the confinement of photogenerated minority carriers. Self-consistent subband calculations have been performed to compute the subband structure of the two-dimensional electron or hole gas which is confined by the space-charge induced potential well at the δ-doping layer. Upon increasing photoexcitation, the increase in density of photocreated electrons in the Be structure is clearly seen in the polarised Raman scattering spectrum by the appearance, and subsequent increase in frequency, of the electron plasmon modes. The electric field at the top GaAs/AlxGa1-xAs interface is monitored by measuring the strength of electric field induced LO phonon scattering.

Electroreflectance (ER) measurements were made on GaAs/AxGal-x As coupled quantum wells at room temperature and at 77K. The close coupling of the narrow (28.3 A) barriers resulted in splitting of the hole and electron subband levels which was observed at both temperatures. As a first approximation, the confining potential was assumed to be due to the barrier alone; electric field effects of the modulating potential and trapped interfacial charge were considered to be negligible. The dominant heavy holeconduction and light hole-conduction transitions observed at 300K were at 1523meV and 1539 meV respectively, although other weaker transitions were also resolved.

The electrical and optical properties of the heterostructure interface between high resistivity ZnSe and the semi-insulating GaAs substrate have been investigated using Transverse Acoustoelectric Voltage (TAV) spectroscopy. From the TAV spectra and the relative change of the TAV amplitude (ΔTAV/TAV), we have found the carrier type and concentration of ZnSe as well as the energy levels of various trap states at the heterostructure interface. The spectral behavior of the ΔTAV/TAV curves varied for samples of different ZnSe epilayer thickness. From the measurements, the surface recombination velocities (SRV's) were calculated. For the pseudomorphic ZnSe films on GaAs, a reduction in the SRV's was measured. As the thickness of the ZnSe film was increased, the various ΔTAV/TAV indicated presence of a large number of interface states due to the introduction of misfit dislocations.

We present the results of pseudopotential calculations on the InAs/GaAs system with and without interlayers of Ge at the interface. We find that the band alignment depends critically on the positioning of the interlayer at the interface and on the number of monolayers substituted. The results can be explained with recourse to a simple model based on charge transfer between the atoms across the interface.

TeraHertz beams of ultrafast pulses of far infrared (λ=40–1000μm, v = 0.3x1012− 7.5×1012Hz) radiation generated via the induced dipole of photogenerated charge within a strong electric field in semiconductors are an emerging spectroscopic technique which incorporates ultrafast optical pulse generation, optoelectronics, and far infrared techniques. Recent results obtained using the trap enhanced field (TEF)[1] effect in the generation of THz beams demonstrate the extended frequency range of these sources and show their importance to time resolved infrared spectroscopy.

The generation of collimated THz radiation within semi-insulating materials is dramatically improved by the extremely large field enhancement near the anode of an electrically biased metal/semi-insulator/metal structure. Our experimental results for semiinsulating GaAs establish an operational regime in which the applied potential is confined to a small region near the anode resulting from a space charge region which exists due to a dramatic change in the number of ionized EL2 traps. The effect, contrary to that observed in trap free or doped materials, is enhanced by optical injection of carriers near the anode, and can be exploited for the efficient generation of ultrafast THz radiation.

Spectroscopy using THz beams allows both the static and dynamic properties, including refractive index, absorption, and photoconductivity, of materials and structures to be accurately measured. The energy range and time resolution of freely propagating subpsec THz pulses have not been previously available.

Variable length semi-insulating GaAs p+-υ-n+ diodes are used to investigate the influence of near contact electric field non-uniformities on the injection of minority carriers. The results show that despite the presence of highly linear IV characteristics, significant nonuniformities in the electric field dominate the device response. The current density through the device is shown to depend on the device length with a power law J ∞ (L)1.0 at a constant bias. The experimental results are compared, with good agreement, to a theoretical model which treats semi-insulating GaAs as a trap-dominated relaxation semiconductor. Electroabsorption measurements are used to observe the slow transients associated with the appearance of near contact field nonuniformities.

We report an optically controlled and space-charge related formation and erasure of large electric fields in the electrode region of CdTe:In. The measurement of the field distribution under different illumination conditions can be explained by a simple model. This nonlinearity has been used in optically controlled switching devices and spatial light modulators.

Transient response times for extrinsic photoconductors under constant voltage bias have been calculated using a variable finite-difference technique. We find that the transient response to a step function increase in photon signal is determined by the build-up of a space charge barrier to counteract out-diffusion and sweep-out. Transient times on the order of 10−2 – 10−4 s are found for 20–500 μm thick detectors under the photon backgrounds typical of infrared astronomy and other low background spectroscopy applications.

A cryogenic extrinsic silicon detector which can detect IR photons of very low energy, i.e., down to about 5.5 meV ( 220 μm) is presented. The mechanism involves photoexcitation of carriers over low energy interfacial work function barriers at p-i or n-i interfaces in a process analogous to the classic photoelectric effect. Estimates for the responsivity and the detectivity for unoptimized commercial samples containing both p-i and n-i interfaces and a sample containing only a single p-i interface are provided by comparison with a silicon composite bolometer. The range of long wavelength thresholds (λt) observed suggests that this approach can be used to tailor detectors for different IR wavelength regions by changing the dopant impurity and the impurity concentration at levels near the metal-insulator transition. Single interface results confirm the possibility of detector optimization using multilayered structures. This should lead to a unique family of uniform monolithic IR focal plane structures with tailorable and tunable response characteristics within virtually all the IR spectral range of interest.

Optical excitation of electrons in an asymmetric double quantum well is theoretically examined. The well is biased to align the excited levels and permit resonant electron tunneling. Emphasis is made on the photoinduced transfer of electrons counter to the bias electric field force. A density-matrix approach is developed to describe optical excitations in the presence of an arbitrary dephasing. The excitation profiles obtained for cases of different dephasing reveal the full range of tunneling coupling between the wells from completely coherent to incoherent (stepwise).

Variations in both the hole and the electron photocurrents in single crystals of mercuric iodide (HgI2) are measured during plastic deformation by shearing over (001) planes. The results demonstrate the presence of electrical interactions between moving “easy glide” dislocations and photo-induced charge carriers in HgI2. It is found that both hole and electron photocurrents are decreased during dislocation motion and that the extent of this effect (and the subsequent recovery after deformation ceases) depends on the applied bias.