Free Carriers In Semiconductors Research Articles

Observation of Optical Orientation of Equilibrium Electrons in N-Type Semiconductors

The first experiments on optical orientation of free carriers in semiconductors were performed recently [1– 5]. Since holes usually do not become oriented in the valence band, owing to rapid relaxation of their spins [6], most experiments were performed on p-type materials and the oriented carriers were the electrons propelled into the conduction band by circularly polarized light. Thus, only the spins of non-equilibrium carriers were oriented, a phenomenon analogous to the optical orientation of excited gas atoms [7]. However, we have previously observed [3] optical orientation also in n-type material. We show in the present paper that this phenomenon is due to optical orientation of equilibrium electrons, i.e., the orientation is realized in the ground state, in analogy with “optical pumping” of atoms [7].

Ab initio theory of free-carrier absorption in semiconductors

The absorption of light by free carriers in semiconductors results in optical loss for all photon wavelengths. Since free-carrier absorption competes with optical transitions across the band gap, it also reduces the efficiency of optoelectronic devices such as solar cells because it does not generate electron-hole pairs. In this work, we develop a first-principles theory of free-carrier absorption taking into account both single-particle excitations and the collective Drude term, and we demonstrate its application to the case of doped Si. We determine the free-carrier absorption coefficient as a function of carrier concentration and we obtain excellent agreement with experimental data. We identify the dominant processes that contribute to free-carrier absorption at various photon wavelengths, and analyze the results to evaluate the impact of this loss mechanism on the efficiency of Si-based optoelectronic devices.

Time-Domain Magnetic Field-Difference Spectroscopy for Semiconductors Using Circularly Polarized Terahertz Pulses

We propose a simple method for sensitive magneto-optic spectroscopy in the terahertz (THz) frequency regime without the need for detecting two orthogonally polarized field components. This simple method is based on the time-domain reflection spectroscopy and employs a single circular polarization state for probing free carriers under a magnetic field that is modulated by a rotating disk with permanent magnets. The evaluated modulation signal directly reflects the Faraday/Kerr ellipticity and optical rotation, which are sensitive to the characteristics of free carriers in semiconductors. We present results of the magneto-optic Kerr spectroscopy on n- and p-Ge substrates with different carrier concentrations and InSb. The experimental results indicate larger Kerr ellipticity and optical rotation near the plasma frequency. Due to the Faraday effect, we also observe an enhanced modulation signal of the THz pulse reflected from the backside surface of the low-carrier-concentration sample. Numerical simulations based on a simple free-carrier model reproduce our experimental results well. Our demonstration will help to advance multipurpose characterization techniques for various semiconductors.

Transport properties of free carriers in semiconductors studied by terahertz time-domain magneto-optical ellipsometry

We have developed a terahertz time-domain magneto-optical ellipsometry to deduce the effective mass, scattering time, density, and type (n or p) of free carriers independently. The parameters are derived from diagonal and off-diagonal components of the complex dielectric tensor obtained by measurements of magneto-optical Kerr effects under a magnetic field of 0.46 T using the generalized Drude model. The derivation of these parameters for n-type InAs wafers with different carrier densities is demonstrated. The carrier density dependence of the effective mass agrees well with previously reported experimental results and theoretical calculations that take into account nonparabolicity of conduction bands.

Time-Resolved Terahertz Spectroscopy of Free Carrier Nonlinear Dynamics in Semiconductors

Nonlinear dynamics of free carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using the intense few-cycle source available at the Advanced Laser Light Source (ALLS). Techniques such as Z-scan and optical-pump/THz-probe are employed to explore nonlinear interactions in an n-doped InGaAs thin film and a photoexcited GaAs sample, respectively. The physical mechanism that gives rise to such interactions is found to be intervalley scattering. A simple Drude-based mathematical model that incorporates the intervalley scattering process is developed and agrees well with the THz response of free carriers in semiconductors.

Free-carrier absorption in nitrides from first principles

We have developed a computationally-tractable first-principles approach (based on density-functional and many-body perturbation theories) to treat the indirect absorption of light by free carriers in semiconductors and insulators and applied it to the technologically important class of group-III nitrides. Indirect absorption by free and impurity-bound carriers, mediated by electron-phonon, charged-defect, and alloy scattering, is an important loss mechanism which may explain the origin of the observed absorption loss in nitride laser devices. The electron-phonon interaction is calculated entirely from first principles, allowing us to validate the commonly used Fr\ohlich approximation. The formalism is quite general and can be applied to other cases where carrier-induced absorption is a concern, such as in transparent conducting oxides.

Infrared optical switch by the use of optically excited free carriers in semiconductors

Infrared light switching was achieved by using the infrared absorption due to the optically excited carriers in a semiconductor. A Nd:YAG laser pulse was used as a gate signal for excitation. A Si wafer was chosen as a switching element, since its band-gap energy coincides with the photon energy of the Nd:YAG laser. The intensity of incandescent infrared radiation was controlled by inducing free-carrier absorption in the Si wafer.

Coherent boson states

An analysis is made of the condition for stimulated precipitation of Bose particles to form a coherent Bose condensate. Experimental observations of the interference of Bose condensates are described. A system of equations is proposed for the dynamics of a generator of coherent atomic beams, i.e. an 'atom laser'. This system of equations takes account of the interaction of a Bose condensate with incoherent particle subsystems. A description is given of experimental observations of stimulated amplification of an exciton bunch. The possibility of generation of a coherent state of excitons by direct binding of free carriers in semiconductors and insulators is considered.

Modulation of submillimeter wave radiation by laser-produced free carriers in semiconductors

When a semiconductor substrate is illuminated by an optical beam whose photon energy exceeds its band-gap energy, free carriers are produced in the substrate, which provides a means of control for the substrate reflectance and transmittance of submillimeter electromagnetic waves. This paper is concerned with submillimeter wave modulation by an optically-excited semiconductor. High-resistivity silicon and GaAs are used as the substrate. Continuous radiation with a wavelength of 214 μm is used as the submillimeter wave. A Q-switched Nd:YAG laser is used to excite the free carriers. The experimental results are presented. Effects of semiconductor substrate material, wavelength of the excitation beam, and incident surfaces of the submillimeter wave and the substrate excitation beam, on the modulation waveforms are analyzed. As an application of the proposed modulation technique, submillimeter wave short pulses with variable duration are successfully generated. © 1998 Scripta Technica. Electron Comm Jpn Pt 2, 80(6): 1–9, 1997

Microwave detection of minority carriers in solar cell silicon wafers

Techniques measuring photoconductive decay by means of microwaves (μ-PCD) can be used to detect free carriers in semiconductors. The instrument as developed at ECN for characterization of the solar cell material is described. The experimental details of two measurement techniques and the theoretical background are discussed. In the decay method the effective mean lifetime of the minority carriers is measured. In the harmonic modulation technique information about the lifetime of the minorities is contained in the phase-shift of the microwave signal relative to the phase of the light intensity. The aim of this research is to determine the bulk mean lifetime of the minority carriers and the surface recombination velocities of solar cell silicon wafers by a non-destructive and contactless technique. Typical experiments will be presented.

Investigation of Deep Levels In X-Ray Detector Material With Photo Induced Current Transient Spectroscopy (Picts)

ABSTRACTDeep levels have a great influence on the recombination behavior of the free carriers in semiconductors. For several years PICTS has been used to investigate the deep levels in high resistivity material such as GaAs or CdTe used in detector applications. An important feature of the PICTS measurements is the analysis of the current transients after pulse excitation. We propose using a new method based on Tikhonov regularization. This method was implemented in the program FTIKREG (Fast Tikhonov Regularization) by one of the authors. The superior resolution of the regularization method in comparison to conventional techniques is shown using simulated data. Moreover, the method is applied to investigate deep levels in CdTe:Cl, SI-GaAs and GaAs:Cr samples used for room temperature radiation detectors. A relation between deep level properties and detector performance is proposed.

Semiconductor device physics and modelling. Part 1: Overview of fundamental theories and equations

The mathematical physics related to semiconductor materials and devices is reviewed, and its relevance and application to the commonly used semiconductor device equations and models is discussed. An overview is given to provide a methodical link between the fundamental theories and equations. The fundamental theories start from the wave and particle properties of electrons and the statistics of free carriers in semiconductors. This, together with the effective mass concept, leads to the energy band structure of semiconductors. Based on the principle of momentum conservation and the free-carrier statistics, we also derive the Boltzmann transport equation, which is considered the most fundamental equation for semiconductor device physics. The widely used drift-diffusion equations are then obtained from the Boltzmann transport equation by using several assumptions such as the relaxation-time approximation and that the semiconductor is isothermal. A summary of the basic equations used in classic device physics is also included.

Resonances of intraband absorption of infrared radiation in semiconductors

The effect of the internal structure of an impurity centre on the bremsstrahlung absorption processes of infrared radiation by free carriers in semiconductors has been analysed. It is shown that the presence of a bound current carrier in the impurity level significantly influences the frequency dependence of the intraband absorption coefficient.

Infrared emission of free carriers in semiconductors below the fundamental absorption edge

It is shown that the IR emission spectrum of free carriers in semiconductors below the fundamental absorption edge ( ω < E g h ; E g is the forbidden band gap) can take the form of a curve with a maximum whose position is determined by the sample thickness, doping level and scattering mechanism in the material. In optically thick crystals ( K λ d > 1; K λ, is absorption coefficient and d is the semiconductor plate thickness) the emissive flux power at the maximum, allowing for reflection, reaches the black body emissive power. With decreasing crystal thickness or doping level (at a fixed temperature) the position of the maximum in the spectrum shifts to the long-wavelength region; at the same time the emission flux power falls off, while the spectral characteristic becomes less selective, mainly due to a decreasing contribution of high-frequency emission. The experiment was carried out on n-type germanium plates (5 × 10 16 ⩽ N d − N a ⩽ 5 × 10 18cm −3; 0.2 ⩽ d ⩽ 10mm) in a spectral range 5 ⩽ λ ⩽ 25 μm at 310 ⩽ T ⩽ 450 K.

New method of measuring relaxation times in semiconductors

A new method of measuring relaxation times of free carriers in semiconductors is proposed and demonstrated. It is based on the pulse duration dependence of free-carrier absorption through the sample. This method has been demonstrated with GaAs and InSb.

Theory of nonlinear optical absorption associated with free carriers in semiconductors

We present calculations of the intensity dependence of the free-carrier absorption in semiconductors by high-intensity light with a wavelength near 10 μm. The paper is divided into two sections. The first section examines the nonlinear absorptive and dispersive properties associated with free-hole transitions in semiconductors, and the second section presents calculations of the nonlinear absorption associated with free-electron intraband transitions in germanium. The dominant free-hole absorption of CO 2 laser light for most p-type semiconductors with a diamond or zinc-blende crystal structure is direct intervalence-band transitions where a hole in the heavy- (or light-) hole band absorbs a photon and makes a direct transition to another band within the valence band structure. The absorption coefficient due to this mechanism is found to decrease with increasing intensity in a manner closely approximated by an inhomogeneously broadened two-level model. We present detailed results for the saturation behavior of germanium as a function of temperature, wavelength, and doping density. Calculated values for the intensity dependence of the index of refraction and low-frequency conductivity are presented. Calculated values of the saturation intensity are also given for most of the other Groups IV and III-V semiconductors. In several n-type semiconductors, the dominant absorption of 10 μm light is free-electron intraband transitions where an electron absorbs a photon and is excited to a state in the same band. The interaction of the electron distribution with the high-intensity light increases the average energy of the electrons and leads to an increase in the free-carrier cross section. For sufficiently high intensities, a significant fraction of the electron density in the interaction region can have an energy greater than the bandgap (relative to the conduction band minimum) by successive one-photon intraband transitions. These hot electrons can relax by creating an electron-hole pair by an impact ionization process. This can lead to the formation of an optically induced plasma and an abrupt increase in the absorption coefficient for light well below the bandgap of the material. Calculated threshold values for the formation of a laser-induced plasma by this impact ionization process are presented for germanium as a function of the lattice temperature and wavelength of the CO 2 laser light.

Calculation of cyclotron resonance linewidth due to ionized impurities and longitudinal optical phonons

Abstract The cyclotron resonance linewidth for free carriers in semiconductors is calculated, starting from the Kubo formula. Ionized impurity and LO phonon scattering are taken into account. The absorption spectrum is investigated in the Relaxation Kernel approach and the scattering is treated in the lowest order Born approximation. For T >15 K, excellent agreement is obtained with the measurements of Matsuda et al. in InSb. No adjustable parameters are introduced.

LO phonon instability in the absorption of radiation by free carriers in semiconductors

LO phonon instability in the absorption of radiation by free carriers in semiconductors

Ballistic heat pulses in crystals

The physics of phonons, or elastic waves, with gigahertz to terahertz frequencies is currently gaining considerable attention. A variety of new phonon techniques has been developed over the past few years as a result of new technologies and materials; they include notable advances in the generation of tunable monochromatic phonon beams at terahertz frequencies, in the creation of “phonon optics” devices such as reflection gratings for ultrasonic waves, and in the interaction of high-frequency phonons with impurities and free carriers in semiconductors. This recent activity was particularly evident at the Third International Conference on Phonon Scattering in Condensed Matter held last year at Brown University.

Quantum transport in semiconductors

A review is made of the quantum effects which are observed in the transport coefficients of semiconductors. Quantization of the free carriers in semiconductors is produced whenever an external pote...