Photoelectricity of β-Agl under sub-bandgap illumination

An experimental study of the spectral, intensity and temperature dependences of the photoconductivity and contact photovoltage of β-AgI single crystals under sub-bandgap illumination is carried out. Both dc and ac photoconductivity experiments are performed and dark-to-light relaxation characteristics of the contact photovoltage are investigated. It is found that the processes under consideration are of a unified character in the spectral region below 2.20 eV, in which the photoconductivity spectrum follows Urbach's rule, temperature behaviour is opposite to that of dark conductivity and dependence on exciting light intensity is linear. A simple model is proposed to explain qualitatively the results obtained, based on the assumption for photoexcited ionic-type conductivity.     

Analysis of the surface photovoltage under sub-bandgap illumination: Effect of the bulk traps

The simple theory of the surface photovoltage induced by photostimulated electron transitions from the bulk impurity levels is given. The relevant calculations show that the surface barrier height, the concentration of the impurity levels and the trapping process at the surface effectively control this phenomenon. Under the proper conditions, a very low excitation level in the bulk can generate significant surface photovoltage signals.     

Influence of sub-bandgap illumination on space charge distribution in CdZnTe detector

The space charge accumulation in CdZnTe crystals seriously affects the performance of high-flux pulse detectors.The influence of sub-bandgap illumination on the space charge distribution and device performance in CdZnTe crystals were studied theoretically by Silvaco TCAD software simulation.The sub-bandgap illumination with a wavelength of 890 nm and intensity of 8×10?8 W/cm2 were used in the simulation to explore the space charge distribution and internal electric field distribution in CdZnTe crystals.The simulation results show that the deep level occupation faction is manipulated by the sub-bandgap illumination,thus space charge concentration can be reduced under the bias voltage of 500 V.A flat electric field distribution is obtained,which significantly improves the charge collection efficiency of the CdZnTe detector.Meanwhile,premised on the high resistivity of CdZnTe crystal,the space charge concentration in the crystal can be further reduced with the wavelength of 850 nm and intensity of 1×10?7 W/cm2 illumination.The electric field distribution is flatter and the carrier collection efficiency of the device can be improved more effectively.     

Stochastic self-oscillations in an extrinsic semiconductor under intense illumination

Summary We show that an extrinsic semiconductor (n-type), containing a deep level from impurity centres, may produce stochastic self-oscillations in the electron density and in the electric field, under intense illumination which ionizes the deep traps and heats the electrons. The feedback mechanism is the dependence of the capture coefficient of the deep centres on the electron temperature and the dependence of the deep energy level on electron temperature and concentration. The conditions for the occurrence of periodic and stochastic self-oscillations are explicitly given.

---

Riassunto Si mostra che in un semiconduttore estrinseco (di tipon), contenente impurezze con un livello profondo, si possono produrre auto-oscillazioni stocastiche nella densità di elettroni in modo da ionizzare le impurezze e riscaldare gli elettroni. Il meccanismo responsabile è la dipendenza della sezione di cattura degli elettroni nel livello profondo della temperatura degli elettroni e la dipendenza dell’energia del livello dalla temperatura degli elettroni e dalla loro densitá. Si danno le esplicite condizioni per l’insorgere di auto-oscillazioni periodiche e stocastiche.

---

Резюме Мы показываем, что собственный полупроводник (n-типа), содержащий глубокий уровень, связанный с примесными центрами, может производить стохастические автоколебания в электронной плотности и в электрическом поле при интенсивном облучении, которое ионизует глубокие ловушки и нагревает электроны. Этот механизм обратной связи представляет зависимость коэффициента захвата для глубоких центров от электронной температуры и зависимость глубокого энергетического уровня от температуры и концентрации электронов. В явном виде приводятся условия для возникновения периодических и стохастических автоколебаний.

     

光伏半导体器件对能量小于禁带宽度光子的响应机理研究

利用光子能量为0.12 eV的10.6 μm连续激光分别辐照了禁带宽度为0.91和0.33 eV的光伏碲镉汞探测器. 实验表明,激光辐照下禁带宽度为0.91 eV的探测器输出正电压,而禁带宽度为0.33 eV的探测器对激光的响应方向却与之相反. 为了研究此现象,利用功率密度一定的10.6 μm激光辐照不同开路电压状态下禁带宽度为0.91 eV的探测器,实验结果证实初始开路电压是产生输出电压反向现象的原因. 对这一机理进一步分析发现,光伏探测器在光子能量小于禁带宽度的激光辐照下,其开路电压是热激发载流子导致的热生电动势和自由载流子吸收导致的晶格热效应共同决定的. 关键词： 能量小于禁带宽度的光子 光伏碲镉汞探测器 热生电动势 晶格热效应     

Slow variations of the surface potential of CdS layers under illumination

Observations have been made of slow changes in the surface potential of CdS layers as a result of illumination, the kinetics and certain other regularities of this phenomenon have been investigated, and a phenomenological theory for the phenomen is presented. It is shown that the disruption of the adsorption equilibrium upon illumination does not play an important role in the mechanism of the surface photoemf, which is determined mainly by changes in the charge on fixed surface traps.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 79–84, December, 1970.     

Near-field investigation of exciton dynamics under semiconductor surface

The near-field images calculation method for the semiconductor surface with the excitons generated by strong focused laser pulse was proposed. Calculation was performed using Green function method in the frame of concept of local field. The main characteristic of the proposed approach is maximal usage of the analytical calculations. The near-field images for the Si surface were studied. Developed approach is universal and could be able to find with experimental data on time-resolved near-field microscopy some parameters of exciton such as diffusion constant, relaxation time, and surface state density.     

Formation of arsenic sulfide on GaAs surface under illumination in acidified thiourea electrolytes

The present article reports the formation of arsenic sulfide films on GaAs by the potentiodynamic polarization in acidified thiourea (TU) electrolytes under photo-illumination. Oxidation of TU competes with the oxidation of GaAs itself and leads to the formation of surface arsenic-sulfide films. Surface chemical composition is investigated by X-ray photoelectron spectroscopy (XPS), demonstrating the formation of As-sulfide as the XPS peaks at binding energies of 42.6 and 162.5 eV for As 3d and S 2p, respectively, are observed. XPS results also show diminishing of Ga species from the surface while As-sulfide is forming. Though, As-sulfide is predominantly formed on the surface, but the inductive coupling plasma-mass spectroscopy (ICP-MS) analysis still shows a preferential dissolution of As ions into electrolytes. These results indicate that Ga ions diffuse into the bulk of the electrode material. The formation of As-sulfide, initially, enhances the photocurrent generation; presumably, due to suppressing electron-hole recombination processes. Further deposition of As-sulfide deteriorates GaAs photoactivity due to retarding light absorptivity because of depositing a thick As-sulfide film. The morphology of the As-sulfide film is characterized by scanning electron microscopy (SEM) that shows the formation of smooth and nonporous films in TU electrolytes acidified by H₂SO₄ of concentration ≥0.2 M. Electrochemical impedance measurements show that GaAs corrosion is limited by the growth and oxidation of the sulfide layer.     

Long-term changes in the surface potential of CdGeP2 single crystals under illumination

A study is made of the variation in the surface potential V_c of p-type CdGeP₂ single crystals during and after illumination. The sign of V_c corresponds to the increase of the work function of the semiconductor surface under illumination. Together with a sharp variation of V_c, a long-term component is observed whose value increases with the increase of the light intensity and the time of exposure of the specimens to light. Slow changes are explained by the presence on the semiconductor surface of an amorphous film, which impedes the electron exchange of the semiconductor with surface states. A model is proposed and a calculation carried out for the long-term kinetics in the small-signal approximation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 51–54, March, 1976.     

Formation of nanostructure on the surface of layered InSe semiconductor caused by oxidation under heating

Variations in the morphology of an InSe surface caused by air oxidation and the influence of the oxide-layered-semiconductor heteroboundary on the photoelectric properties of the In₂O₃-InSe structure were studied. It is found that an ordered nanosized heteroboundary can form as a result of oxidation of the layered InSe semiconductor and that this heteroboundary determines the high photosensitivity of the In₂O₃-InSe heterostructure in the spectral region of exciton absorption at room temperature.     

Chaotic potential on semiconductor boundary under conditions of the partial self-organization of the surface ion charge

The possibility of varying the parameters of electrostatic chaotic potential on a semiconductor surface with the association of point defects is investigated. Two models of surface dipole structures are considered. The amplitude and type of chaotic potential spread are determined under conditions of the partial self-organization of the ion charge. A reduction in the degree of chaos is obtained that depends on a structural parameter of a system of dipoles (their surface concentration).     

Self-organization of ordered nano- and microstructures on the semiconductor surface under the action of laser radiation

Experimental results on the formation of ordered nano- and microstructures on the surface of semi-conductors under the action of laser pulses with various durations and fluences are interpreted from the unified view point of the theory of the defect-deformation (DD) instability of surface relief. A universal linear dependence of the period of the structures on the thickness of the subsurface layer enriched with mobile point defects and formed due to the laser action and occurrence of two scales of modulation of the surface relief are established and described. The structure symmetry and its evolution with an increasing laser fluence and magnitude of external anisotropic stress are described. Similarities with the formation of nanostructures under ion-beam irradiation and electrochemical etching are revealed and discussed within the framework of the DD instability theory.     

Time-dependent chemisorption on a semiconductor surface

The microscopic aspects of interfacial growth are only crudely accessible to experiment. We have therefore developed a technique for computer-experimental studies of atoms and molecules deposited on solid surfaces, in which forces are determined from the electronic structure rather than atomic potentials. Here we report simulations of Cl, O, Se, N, As, Si, Ge, Al, Ga, Zn, Hg, Cu, and Au on the (1 1 0) surface of GaAs. The results exhibit non-trivial variety in both the dynamics and final bonding sites.     

Surface photovoltage in semiconductors under sub-band-gap illumination: continuous distribution of surface states

Surface photovoltage spectra in semiconductors are analyzed when the sub-band-gap illumination induces the electron transitions from surface states to the conduction band under the assumption that distribution of surface states is continuous. From analysis performed it follows that the fictitious densities of surface states can be induced due to the wavelength dependence of the photoionization capture cross-section of surface states for electrons and by the electron recombination capture cross-section of surface states which depends on the energy position of surface states in the energy gap. The high illumination intensity (laser illumination), which makes completely empty the surface states, can eliminate the fictious surface states when the density of surface states is not very large, the temperature of measurements is low, and the surface potential barrier is high. Received: 24 June 1998 / Accepted: 29 March 1999 / Published online: 14 June 1999     

On the gate capacitance of MOS structures of kane-type semiconductors under magnetic quantization

An attempt is made to formulate the gate capacitance of MOS structures of Kane-type semiconductors under magnetic quantization, without any approximations of weak or strong electric field limits, on the basis of the fourth-order effective mass theory and taking into account the interactions of the conduction, light-hole, heavy-hole, and split-off bands. It is found, taking n-channel Hg_1–x Cd _x Te as an example, that the gate capacitance exhibits spiky oscillations with changing magnetic field, which is in qualitative agreement with experimental observations, reported elsewhere, in MOS structures of the same semiconductor. The corresponding results for n-channel inversion layers on parabolic semiconductors are also obtained from the expressions derived.     

Photoluminescence as a tool for the study of the electronic surface properties of gallium arsenide

Electronic surface parameters of GaAs have been determined from a qualitative and quantitative analysis of the relative photoluminescence intensity at 300 K. Characteristics of etched (100) surfaces ofn- andp-type liquid phase epitaxial GaAs have been found to be governed by negative surface charges. A density of charged surface states of about 10¹² cm⁻² and a band bending of 0.59 eV have been found forn-type material with an electron concentration of 1.1×10¹⁷ cm⁻³. Forp-type samples with hole densities ranging from 6×10¹⁵ to 4.3×10¹⁸ cm⁻³ the estimated density of negatively charged surface states was below 2×10¹¹ cm⁻², and the band bending was not more than a few kT.     

Micropore formation by the electrochemical etching of a semiconductor surface under local electrical-breakdown conditions

It is shown that local tunneling breakdown causes selective etching of the surface of a semiconductor electrode. It is shown that the effect of the axially directed etching of micropores in heavily doped semiconductors stops at an anode voltage, when the depletion layer of the inner end face of the pore becomes equal to the depletion layer of the planar surface. In lightly doped semiconductors, lateral etching of pore boundaries is controlled by the initiation of cascade ionization during avalanche breakdown.     

Self-assembled molecular corrals on a semiconductor surface

Nano-corrals for capturing surface electrons are of interest in molecular electronics. Here we show that haloalkane molecules, e.g., 1-chlorododecane, physisorbed on Si(1 1 1)-(7 × 7) self-assemble to form dimers stable to 100 °C which corral silicon adatoms. Corral size is shown to be governed by the haloalkane chain-length. Spectroscopic and theoretical evidence shows that the haloalkane dimer induces electron transfer to the corralled adatom, shifting its energy levels by ∼1 eV. Isolation of a labile pre-cursor points to a model for corral formation which combines mobility with immobility; monomers diffusing in a mobile vertical state meet and convert to the immobile horizontal dimers constituting the corrals.     

Nanopore formation on low-doped p-type silicon under illumination

Porous silicon layers were elaborated by anodization of highly resistive p-type silicon in HF/ethylene glycol solution under front side illumination, as a function of etching time, HF concentration and illumination intensity. The porous layer morphology was investigated by scanning electron microscopy (SEM). The illumination during anodization was provided by a tungsten lamp or lasers of different wavelengths. Under anodization, a microporous layer is formed up to a critical thickness above which macropores appear. Under illumination, the instability limiting the growth of the microporous layer occurs at a critical thickness much larger than in the dark. This critical thickness depends on HF concentration, illumination wavelength and intensity. These non-trivial dependencies are rationalized in a model in which photochemical etching in the electrochemically formed porous layer plays the central role.     

Low-dimensional nanostructures and fullerene films on semiconductor surface

The morphology and atomic structures of C₆₀ fullerene films on a Bi(0001)/Si(111)-7 × 7 surface and adsorption of fluorofullerene C₆₀F _x molecules on a Si(111)-7 × 7 surface have been studied by scanning tunneling microscopy/spectroscopy and low-energy electron microscopy under ultra high-vacuum conditions. It has been shown that initial nucleation of C₆₀ islands on the surface of an epitaxial Bi film occurs on double steps and domain boundaries, while tunnel spectra do not exhibit any significant charge transfer to the lowest unoccupied molecular orbital states. Fluorofullerene molecules allow local (at the nanoscale level) modification of Si surface through local etching.