Dae M. Kim

Theory of conduction in polysilicon: Drift-diffusion approach in crystalline-amorphous-crystalline semiconductor system—Part I: Small signal theory

A theory of conduction in polycrystalline silicon is presented. The present approach fundamentally differs from previous theories in its treatment of the grain boundary. This theory regards the grain boundary as amorphous semiconductor in equilibrium contact with crystalline grain. The model explains the electrical properties of polysilicon in terms of the electronic and structural parameters of the material and is in excellent agreement with the experimental data. The formulation is applicable for arbitrary grain size, temperature, doping concentration, and applied voltage. Specifically, the temperature dependence of resistivity is explained in terms of conduction channels inherent in the amorphous grain boundary. Also, this paper explicitly compares the previous emission theories with the present model in terms of voltage partition scheme and I - V predictions.

Pulsed laser heating of silicon: The coupling of optical absorption and thermal conduction during irradiation

Transient laser annealing of semiconductors is examined extensively and the rise in lattice temperature specifically attained in silicon under the influence of a high‐power laser beam irradiation is characterized analytically over a wide range of laser wavelengths, intensities, and durations. For the case of silicon, with increasing lattice temperature the optical absorption coefficient is drastically enhanced, while the thermal conductivity is considerably reduced. These two temperature‐dependent material parameters are, therefore, strongly coupled during the laser beam irradiation, and the effects of this coupling on the ensuing lattice‐temperature rise are examined comprehensively. Specifically, the threshold pulse energy for the onset of surface melting is analytically calculated as a function of both material parameters and operating laser beam characteristics.

Nonlinear dynamic theory for photorefractive phase hologram formation

A nonlinear dynamic theory is developed for the formation of photorefractive volume phase holograms. A feedback mechanism existing between the photogenerated field and free‐electron density, treated explicitly yields the growth and saturation of the space‐charge field in a time scale characterized by the coupling strength between them. The expression for the field reduces in the short time limit to previous theories and approaches in the long time limit the internal or photovoltaic field. Additionally, the phase of the space‐charge field is shown to be time dependent.

Conduction in polycrystalline silicon: Diffusion theory and extended state mobility model

We present a new model for conduction in polycrystalline silicon based on an extended state mobility in the disordered grain boundary and the diffusion theory of current. This analysis for the first time satisfactorily explains-without the use of scaling or artificial factors-experimental data on current density, mobility, resistivity, and the activation energy for carriers in polysilicon. An attractive feature of this theory is that it provides simple expressions for J, µ, and ρ which may even be derived from an equivalent circuit model. Also, these expressions reduce, within appropriate limits, to the corresponding terms for single crystal or amorphous material.

Characterization of iron‐doped lithium niobate for holographic storage applications

A detailed study of eight systematically chosen Fe : LiNbO3 crystals is presented. Correlation between the photorefractive sensitivity and various chemical properties of Fe : LiNbO3 is investigated in order to ascertain optimum performance of the crystals in holographic storage and display applications. Concentrations of Fe2+ and Fe3+ ions have been determined from optical and EPR spectra, while impurities have been detected from x‐ray‐emission and infrared spectra. Particular emphasis is placed on investigating the dependence on Fe2+ and Fe3+ ion concentrations of the photorefractive sensitivity. The photorefractive sensitivity is shown to depend primarily on the concentration of Fe2+ ions in Fe : LiNbO3. This fact seems to suggest that Fe2+ ions are the impurity centers responsible for the photorefractive effect in Fe : LiNbO3. Spectral dependence of the photorefractive sensitivity and its modification due to oxygen annealing are also reported. Our results indicate that an unannealed Fe : LiNbO3 crystal...

Laser heating of semiconductors—effect of carrier diffusion in nonlinear dynamic heat transport process

An analytic theory is presented to describe the nonlinear dynamic heat transport process in a semiconductor irradiated by a pulsed laser beam. The input rate of laser energy to the lattice is sensitively influenced by the ambipolar diffusion of the dense, laser‐produced excess charge carriers. Additionally, the high heating rate of the laser beam significantly changes the material transport coefficients during the pulse. This nonlinear laser beam‐solid interaction is examined from the viewpoint of the transport process, using a parametrized perturbation technique in Green′s function formulation. An explicit, analytical solution of the lattice temperature rise is presented and the threshold pulse energy for the onset of surface melting for the case of amorphous Si is calculated as a function of laser beam intensity as well as the carrier diffusion length. Our results are compared with both the melting and nonthermal models for laser annealing.

Theory of conduction in polysilicon: Drift-diffusion approach in crystalline-amorphous-crystalline semiconductor system—Part II: General I-V theory

A general model for conduction in polysilicon is presented for arbitrary applied voltage. The model incorporates the effect of mobile carrier redistribution under bias and accounts for the high field switching in amorphous grain boundary. Microscopic mobilities used for describing the carrier transport provides a physical basis for introducing the grain voltage across the unit cell of polysilicon. The voltage, in turn, distributes itself to preserve a constant current density therein. This new criterion yields a new voltage partitioning scheme, and a general expression for the corresponding current is derived in terms of pertinent system parameters.

Study of the equivalent electron drift field characteristics in LiNbO3 by phase holography

An analysis of the diffraction efficiency of photorefractive holograms in ferroelectric crystals is shown to provide a novel technique for determining the nature and magnitude of the equivalent electron drift field. For a Fe‐doped lithium niobate crystal (0.05% per mole) we find that the total field consists of an intensity‐independent internal field of 8.5 kV/cm and a photogenerated field given by the conversion factor 1.4 ×106 V cm/W.

Eigenfunction analysis of mode‐locking process

An active mode‐locking process in a homogeneously broadened medium is discussed by using an eigenfunction analysis of a time‐varying strongly interacting closed‐loop system. This direct time domain analysis is shown to yield a complete set of eigenfunctions or supermodes for steady‐state pulse configurations. The kinetics of generated pulses is described in this eigenvector space. The result shows that, depending on the buildup time, the string of detected pulses can differ substantially among themselves. Also, the possibility of using this time domain approach to analyze time‐tailoring pulse envelopes is pointed out.

Erratum: A general analytic technique for nonlinear dynamic transport processes during laser annealing

We present an approximate analytic technique capable of describing the dynamics of nonlinear heat and mass diffusion. The technique is based on using adiabatic approximation in the Green’s‐function formulation and is illustrated for the case of heat transport during pulsed and cw laser annealing of Si. In particular, the heating efficiency is discussed explicitly as a function of pulse intensity for a fixed pulse energy.