Nonlinear transport in far-from-equilibrium semiconductors

Summary Using a nonlinear transport theory, derived from the nonequilibrium statistical operator method, we obtain the equations that govern the evolution of the nonequilibrium state of a highly photoexcited direct-gap polar semiconductor, and its nonequilibrium mobility coefficient. It provides an analytic method that allows for a deep physical insight into the influence of the nonequilibrium irreversible evolution of the plasma in the semiconductor on its transport properties. We demonstrate that, under quite general conditions, the strong dependence of the momentum and energy relaxation times on the irreversible evolution of the macroscopic nonequilibrium state of the system results in the existence of a structured transient mobility,i.e. one with maxima and minima, with or without overshoot. A criterion for the occurrence of this structure is derived as well as several general properties of the ultrafast transient are discussed. The authors of this paper have agreed to not receive the proofs for correction.     

Thermoelectric power,photo and dark conductivity of doped amorphous silicon

Summary Conductivity, photoconductivity and thermopower measurements have been made as functions of temperature on a series of p- and n-type a-Si: H films prepared by a reactive, evaporation method. The results have been fitted with a model including two conduction paths: in the high-temperature range through extended band states and in the low-temperature range as hopping in an impurity band, whose density of states increases with increasing dopant content. A shift of the Fermi level up to 0.4 eV towards the mobility edges for both types of dopants has been found. Moreover, phosphorus doping radiply increases the photoconductivity values by about two orders of magnitude, while boron incorporation causes considerable reduction. This work was supported in part by GNSM-CNR, MPI and CNR ?Progetto Finalizzato Energetica?.     

On the generalized diffusion theory of current transport in isotype heterojunctions

Summary The generalized diffusion theory of current transport inn-n isotype heterojunctions enunciated previously has been extended to the realistic case of there being two different electron mobilities in the two semiconductors forming then-n heterojunction. This extended theory too reduces to the diffusion theory of current in metal-semiconductor contacts under the appropriate limiting conditions in the same way as the previous theory does. The current-voltage characteristics forn-n heterojunctions according to this extended theory would be different from those according to the previous theory if the values of all the parameters involved such as Λ _nn C, ψ_D1, ψ_D2,V ₁,V ₂, etc. could be calculated rigorously. However, utilizing the values of some parameters such as Λ _nn andC calculated by means of approximate expressions derived under these limiting conditions, one would evaluate current-voltage characteristics that are the same as in the previous theory. A single curve representing the current-voltage characteristic of any particularn-n isotype heterojunction can also be plotted by using a formula derived previously under zero-current conditions, in conjunction with the appropriate expressions derived in the present paper. In appendix B an alternative expression for the current density in an-n isotype heterojunction is also derived in the framework of the generalized diffusion theory.

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Riassunto La teoria della diffusione generalizzata del transporto di corrente nelle eterogiunzioni di isotropin-n enunciata in precedenza è stata estesa al caso realistico in cui vi sono due diverse mobilità di elettroni nei due semiconduttori che formano l'eterogiunzionen-n, Anche questa teoria estesa si riduce alla teoria di diffusione della corrente nei contatti dei semiconduttori metallici nelle condizioni limitanti appropriate nello stesso modo della teoria precedente. Le caratteristiche del voltaggio di corrente per le eterogiunzioni n-n secondo questa teoria estesa sarebbero differenti da quelle secondo la teoria precedente se i valori di tutti i parametri coinvolti come Λ _nn ,C, ψ_D1, ψ_D2,V ₁,V ₂, ecc. potessero essere calcolati rigorosamente. Comunque, utilizzando i valori di alcuni parametri come Λ _nn eC calcolati per mezzo di espressioni approssimate dedotte in queste condizioni limitanti, si valuterebbero le caratteristiche del voltaggio di corrente uguali a quelle delle teoria precedente. Si può anche tracciare una singola curva rappresentante la caratteristica del voltaggio di corrente di ogni particolare eterogiunzione di isotopin-n usando una formula dedotta precedentemente in condizioni di corrente zero, assieme alle espressioni appropriate dedotte in questo lavoro. Nell'appendice B si deduce anche un'espressione alternativa per la densità di corrente in un'eterogiunzione di isotopin-n nel modello della teoria della diffusione generalizzata.

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Рэзюме Обобщенная теория диффузэии для переноса тока в n-n гетеропереходе, развитая ранее, обобщается на реалистический случай, когде в двук полупроводниках, образующих n-n гетеропереход, электроны имеют две различные подвижности. Предложенная обобщенная теииая теория сводится к теории диффузии для тока в контакте металл-полупроводник при определенных ограничивающих условиях, как и в случае предыдущей теории. Вольт-амперная характеристика для n-n гетероперереходов, согласно предлагаемой обобщенной теории, будет отличаться от вольт-амперной характеристики предыдущей теории, если все парамеры теории Λ _nn , С, ψ_D1, ψ_D2,V ₁,V ₂ и т.д, могут быть вычислены строго. Однако, используя для некоторых параметров, таких как Λ _nn и С, приближенные выражения, вычисленные в рамках вышеуказанных ограничивающих условий, получаются вольт-амперные характеристики, совпадающие с результатами предыдущей теории. Приводятся кривые для вольт-амперных характеристик для любого n-n гетероперехода. В Приложении П в рамках обобщенной теории диффузии выводится альтернативное выражение для плотности тока в n-n гетеропереходе.

     

Influence of quantization of band states on the thermoelectric power in II–VI semiconductors

Summary An attempt is made to present a simplified analysis of the thermoelectric power in II–VI semiconductors in the presence of quantization of band states under different physical conditions. The thermopower decreases with increasing electron concentration in different manners, which is a direct consequence of band state quantization. We have plotted the thermopower with various other physical variables and the same power exhibits different types of oscillations in low-dimensions, respectively. The well-known results for parabolic energy bands have also been obtained as special cases of our generalized formulations under certain limiting conditions. In addition, the theoretical results are in quantitative agreement with the experimental data.     

Photoelectronic properties of PbI2 using dye-lasers

Summary Pure PbI₂ single crystals have been grown by the zone-refining technique and the spectral distribution of the photocurrent by using different dye-lasers has been recorded withE‖c andE⊥c. The fundamental absorption edge appears at 2.36 eV (E⊥c) and is found to be dichromatic in nature with a splitting of 0.05 eV. The decay pattern of the photocurrent at different voltages, temperatures and intensities of illumination has been used to calculate the lifetimes of charge carriers. The photocurrent-voltage characteristics have been used to calculate the trap concentration and mobility of the charge carriers. The lux-ampère characteristics at the band edge frequency have been studied at different applied voltages.

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Riassunto Monocristalli puri di PbI₂ sono stati accresciuti con la tecnica di raffinazione a zone ed è stata registrata la distribuzione spettrale della fotocorrente con l'uso di diversi laser a colore conE‖c eE⊥c. Il bordo di assorbimento fondamentale appare a 2.36 eV (E⊥c) e appare essere dicromatico in natura con una separazione di 0.05 eV. Lo schema di decadimento della fotocorrente a diversi voltaggi, temperature ed intensità d'illuminazione è stato usato per calcolare le vite medie dei portatori di carica. Le caratteristiche di voltaggio-fotocorrente sono state usate per calcolare la concentrazione di trappole e la mobilità dei portatori di carica. Le caratteristiche lux-ampère alla frequenza del bordo della banda sono state studiate con diversi voltaggi applicati.

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Резюме Выращивается чистый монокристалл PbI₂. Регистрируется спектральное распределение фототока, используя различные лазеры на красителях, в случаеE‖c иE⊥c. Фундаментальный край поглощения возникает при 2.36 эВ (E⊥c). Обнаружена дихроматичность с расщеплением 0.05 эВ. Кривые затухания фототока при различных напряжениях, температурах и интенсивностях облучения используется для вычисления времени жизни носителей заряда. Зависимость фототока от напряжения используется для вычисления концентрации ловушек и подвижности носителей зарядов. При различных приложенных напряжениях исследуются зависимости фототока от интенсивности облучения при частотах, соответствующих краю зоны.

     

Energy loss measurements and electronic properties of thianthren

Summary The dielectric and electronic properties of thianthren (C₆H₄(S)₂H₄C₆) are determined by means of optical reflectivity, absorption measurements and reflection electron energy loss spectroscopy. The experimental results are interpreted on the basis of a Complete Neglect of Differential Overlap (CNDO) calculation used in three different parametrization schemes. Emphasis is laid on the discussion of the problems which generally affect the analysis of electron energy loss spectra, and a procedure to obtain the complex dielectric function from electron energy loss measurements performed in the reflection mode is suggested. Work partially supported by the MURST through the GNSM.     

Electrical and optical properties of CuIn5S8 single crystals

Summary Several transport and optical properties have been studied onn-type CuIn₅S₈ single crystals. The energy gap at 0 K was determined from the electrical measurements to be 1.4 eV. An anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions ink-space. An optical-absorption band was found in an infrared region of (1÷1.6) μm and was attributed to the transitions from the lowest conduction band situated along the [100] directions to an upper conduction band. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Effect of optical-phonon scattering on the energy distribution of hot electrons in semiconductors at low temperature

Summary The variational method is used in finding the solution of the transport equation for a system of hot electrons inn-Ge atT=20 K in the presence of high electric field. The role of the emission of optical phonons by hot electrons together with the effect of electron capture by repulsive centres on the formulation of the distribution function are studied. It is shown that the emission of optical phonons plays a dominant role in the formulation of the distribution. The influence of electron capture is very small, it may become appreciable at higher trap concentrations. The obtained distribution function is then used in calculating the capture rate of electrons by negatively charged centres. It is shown that the capture rate increases with electric field.

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Riassunto Il metodo variazionale è usato per trovare la soluzione dell'equazione di transporto per un sistema di elettroni caldi inn-Ge aT=20 K in presenza di grande campo elettrico. Si studiano il ruolo dell'emissione di fononi ottici da elettroni caldi insieme con l'effetto della cattura degli elettroni da centri di repulsione sulla formulazione della funzione di distribuzione. Si mostra che l'emissione di fononi ottici svolge un ruolo dominante nella formulazione della distribuzione. L'influenza della cattura degli elettroni è molto piccola, può diventare apprezzabile a piú grande concentrazione di trappole. La funzione di distribuzione ottenuta è quindi usata nel calcolare il rapporto di cattura degli elettroni da centri a carica negativa. Si mostra che il rapporto di cattura cresce col campo elettrico.

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Резюме Используется вариационный метод для нахождения решения транспортного уравнения для системы горячих электронов вn-Ge приT=20 К при наличии сильного электрического поля. Исследуется роля испускания оптических фононов горячими электронами и эффект эахвата электпонов отталкивающими центрами при определении функции распредения. Показывается, что испускание оптических фононов играет доминируюэую роль при образовании функции распределения. Влияние захвата электронов очень мало и становится существенным при более высоких концентрациях ловушек. Полученная функция распределения используется при вычис-лении интенсивности захвата злектпонов отрицательно заряженными центрами. Показывается, что интенсивность захвата увеличивается с возрастанием электрического поля.

     

The thermoelectric power in superlattices of III–V semiconductors with graded structures under strong magnetic quantization

Summary An attempt is made to formulate the thermoelectric power under strong magnetic quantization (TPM) in superlattices (SLS) of III–V semiconductors with graded structures and to compare the same with that of the forming materials. It is found, taking Ga_0.8In_0.14P_0.78Sb_0.22/GaAs SL as an example, that the TPM increases with increasing quantizing magnetic field and decreases with increasing electron concentration respectively in an oscillatory manner. The TPM in SL with graded structures is greater than that of the constituent bulk materials for III–V SL.     

On the thermoelectric power in quantum well of A 3 II B 2 V semiconductors in the presence of a classically large magnetic field

Summary An attempt is made to study the thermoelectric power of the carriers in quantum well of A ₃ ^II B ₂ ^V semiconductors, taking Cd₃As₂ quantum well as an example. It is found, on the basis of newly derived 2DE−k _s dispersion relation by including various types of anisotropies in the energy spectrum that the thermoelectric power decreases with increasing electron concentration and decreasing film thickness respectively. In addition, the corresponding well-known results for bulk specimens of isotropic parabolic energy bands are also obtained from the expressions derived.     

Molecular bonding in the conduction states of the Mg2X (X=Si,Ge, Sn) semiconductors

Summary The chemical bond in the electron-deficient semiconductor series Mg₂X (X=Si, Ge, Sn) is analysed in the pseudopotential self-consistent local-density scheme. Hard-core atomic potentials are used to investigate the physical properties of these compounds and to show the regularities peculiar to the periodic table. The ionic character of these materials is studied and related to the scaling electronegativities of their anions. Our results, in good agreement with the experiment, are discussed in order to explain the molecular behaviour of the bonding conduction states.

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Riassunto Si studia il legame chimico nella serie dei semiconduttori mancanti di elettroni Mg₂X, dove X=Si, Ge e Sn, con il metodo dello pseudopotenziale autocoerente nello schema del funzionale densità locale. Le proprietà fisiche dei composti in esame sono studiate utilizzando potenziali atomici che permettono di evidenziarne le caratteristiche di periodicità. Il carattere ionico di questi materiali è correlato all'elettronegatività degli anioni. I nostri risultati, in buon accordo con l'esperimento, sono discussi in modo da spiegare il comportamento molecolare degli stati di conduzione.

     

Electron emission from sharp points in liquid argon

Summary Electron charge injection from a point electrode in gaseous and in liquid argon has been observed and its characteristics studied. In liquid argon, as we have observed, the charge dependence on applied voltage is governed by space charge limitation. The emission was found to take place (with the electrode in liquid phase) not continuously but as a pulsed injection. The pulsing rate has been studied as a function of the voltage, time and electrode separation. The measurements done so far have been performed in a closed environment with an impurity content of argon of the order of some tenth of p.p.b. (oxygen, CO₂, etc.). Possible applications in liquid-argon purification and particle detection is presented. The authors of this paper have agreed to not receive the proofs for correction.     

Harmonic generation in keldysh-type models

Summary We shall investigate two models of harmonic generation in the spirit of the Keldysh theory and we shall apply these models to hydrogen, for which more exact data for the rates of this process are available. Although our models correctly describe the angular distribution, the intensity dependence (in the perturbation regime) and the selection rule for odd harmonics, they badly fail in their predictions of the correct order of magnitude of the rates, in particular, of the formation of a plateau. These latter findings disagree with the results of a similar model calculation by Beckeret al. for a δ-function potential. The author of this paper has agreed to not receive the proofs for correction. This research was supported in part by the Natural Sciences and Engineering Research Council of Canada.     

Magnetoconductance oscillations and hall current for a two-dimensional electron gas with harmonic confining potential

Summary In the present paper we calculate exactly the density of states of an ideal 2DEG in uniform magnetic and electric fields with parabolic confining potential. From the density of states, we obtain the magnetoconductance oscillations. Consequently, we get the Hall current by means of the Feynman-Hellman theorem and study its quantization.

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Riassunto In questo lavoro si calcola esattamente la densità degli stati di un ideale 2DEG in campi uniformi magnetici ed elettrici con potenziale confinante parabolico. Dalla densità degli stati si ottengono le oscillazioni di magnetoconduttanza. Di consequenza si ricava la corrente di Hall per mezzo del teorema di Feynman-Hellman e si studia la sua quantizzazione.

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Резюме В этой работе мы проводим точное вычисление плотности состояний идеального даумерного электронного газа в однородных магнитных и электрических полях с параболическим удерживаущим потенциалом. Из плотности состояний мы получаем осцилляции магнитной проводимости. Таким образом, мы определяем ток Холла с помощью теоремы фейнмана-Гелмана и исследуем его квантование.

     

Electronic-transport properties of unhydrogenated amorphous gallium arsenide

Summary Hall mobility, μ_H, and electrical conductivity, σ, of unhydrogenated amorphous-gallium-arsenide films, prepared by r.f. sputtering, have been measured. Conductivity as a function of temperature shows a variable-range hopping mechanism atT<260 K, while at high temperature, conductivity and Hall mobility are both thermally activated. The results are interpreted in terms of the presence of defect complexes due to an excess of Ga. The stoichiometry and the structure of the films are used to explain the behaviour and the values of μ_H. The values of the activation energy of the conductivity seem in agreement with theoretical calculations on the position of electronic states created by defect complexes in the mobility-gap of a-GaAs.     

Physical properties of hydrogenated amorphous gallium arsenide

Summary The elemental composition and the optical properties of hydrogenated amorphous GaAs prepared by r.f. reactive sputtering at different hydrogen and argon pressure and substrate temperature have been determined. From the dependence of the absorption coefficient on photon energy the optical gap has been deduced according to the Tauc law. The data obtained for stoichiometric samples are compared with similar data obtained by different authors. The influence of various deposition parameters on stoichiometry and on the optical properties is briefly discussed.     

Effect of pressure on turnover behaviour inI–V characteristic curves in GeTeSe chalcogenide glass

Summary Thick layers of GeTeSe chalcogenide glass have been prepared and subjected to conduction measurements under the effect of both temperature and pressure. The results of theI–V characteristics exhibit transition from high-resistance state to differential negative resistance state through a turnover point. The application of uniaxial pressure shows the similar effect of temperature on that behaviour. Both current and voltage at the turnover point depend on pressure and ambient temperature. The rise of temperature in the conduction path due to joule heating and application of uniaxial pressure as well as the reduction in the energy gap width (β=2.87·10⁻¹²eV/N m⁻²) are estimated and discussed at the turnover point. This behaviour is explained according to the orientation of dipoles randomly dispersed in viscous amorphous matrix.     

Interfacial barrier contribution in Na2B4O7 glass system

Summary Frequency domain measurements on Na₂B₄O₇ and Na₂B₄O₇(95%)-V₂O₅(5%) glass systems have been carried out in the frequency range 10⁻² to 10⁴ Hz and temperature range 300 to 450 K. The complex capacitance has shown fractional power law dependences on frequency which obey the generalized Maxwell-Wagner relationships. The activation energy is found to increase with an increase in V₂O₅ composition.     

Transport properties of a nanotube-based transistor

Transport properties of doped nanotube-based double junctions forming a nanotransistor are investigated within the tight binding formalism. The effects of doping, gate length and gate-source hopping have been considered. It is found that in addition to the importance of rotational symmetry in determining transport properties, large gains can be achieved for semiconducting doped tubes. Received 30 November 2000     

Effects of a finite screening length on the absorption of electromagnetic waves

When an electromagnetic wave impinges on a semiconductor or ionic conductor having a sizeable screening length, it induces diffusion currents in addition to the ohmic currents, which affects the propagation in heterostructures or composite media involving such materials. In the simple geometries and in the low frequency regime studied here, the absorption may be either enhanced or reduced, depending on the parameters, and effects precluded for metals are predicted: extinction of the reflection by a plane wall, complete absorption of an electric multipolar wave by a sphere, disappearance of the scattering by a small sphere, vanishing of both reflection and transmission coefficients for a slab. If the screening length is larger than the skin depth, a slab with intermediate thickness may have a large transparency, and a thick piece of material is expected to be cooled down by the wave near the interface and overheated deeper inside. Received: 29 July 1997 / Revised: 24 November 1997 / Accepted: 20 February 1998