Semiconductor Cathode Research Articles

Flexible low-dimensional semiconductor field emission cathodes: fabrication, properties and applications

We present an overview on the fabrication, properties, and applications of flexible field emission cathodes based on low-dimensional semiconductor nanostructures.

Carrier concentration-dependence of the Nottingham cooling of an n-type silicon semiconductor

The authors investigated the Nottingham effect of an n-type silicon semiconductor cathode in a quest for a practical solid-state cooler. The dependence of field emission cooling on the carrier concentration n in the range from 1015 to 1020 cm -3 was analyzed. The potential profile was obtained for a spherical tip with a radius R = 0.5 nm and was used to calculate the energy exchange Δe and the cooling power density Γ as a function of n and the temperature T. The energy exchange Δe was found to remain almost constant for 1019 cm -3 and decrease slowly as n further increased. In contrast, the current density j increased continuously with increasing n. As a result, the cooling power density Γ increased with increasing n, more rapidly at higher T. For n = 1016, 1018, and 1020 cm -3, a bias of V = 6.0 volts yields Γ = 0.49, 49, and 3202 watts/cm2 at 600 K and 2.0, 201, and 16560 at watts/cm2 at 900 K, respectively. This predicts a field emission cooling for micro-electronic devices even with a bias equivalent to several dry cells.

Atomic-size tip enhanced cooling of field emission from the n-type silicon semiconductor

The Nottingham effect of an n-type silicon semiconductor tip was theoretically investigated for use as a practical solid state cooler. The vacuum potential was obtained in the form which explicitly included the semiconductor cathode geometry. This leads to a relatively exact dependence of the energy exchange Δε and the cooling power density Γ on the geometry of the device. By systematic calculations of the field emission cooling, Γ was obtained as a function of the bias V, the tip radius R, and temperature T. The current density j increased with decreasing R at fixed V and T. The Δε increased with decreasing R at fixed j and T. As T increased, both Δε and Γ increased considerably. When an atomic-size silicon tip was taken, a meaningful cooling was obtained at V as small as several volts. At V = 6.8 V, a sharp tip of R = 0.5 nm yielded the maximum Γ = 250, 1941, and 6148 W/cm2 at T = 300, 600, and 900 K, respectively. This implies that an optimized configuration of an n-Si cathode produces a useful field emission cooling for micro-electronic devices with a low bias.

Measurements of copper and cesium telluride cathodes in a radio-frequency photoinjector

Radio-frequency (rf) photoinjectors are commonly used to generate intense bright electron beams for a wide range of applications, most notably as drivers for X-ray Free-Electron Lasers. The photocathode, mounted inside an rf gun and illuminated by a suitable laser, thereby plays a crucial role as the source of the electrons. The intrinsic emittance and the quantum efficiency of the electron source are determined by the properties of the photocathode's surface material. We present measurements of the intrinsic emittance and the quantum efficiency performed with copper and cesium telluride cathodes in the same rf photoinjector, thus comparing, for the first time, the performance of metal and semiconductor cathodes under the same conditions. Our results are consistent with theoretical expectations and show that the difference in intrinsic emittance for the two types of material is not significant in view of accelerator applications. We conclude that cesium telluride photocathodes provide a much higher quantum efficiency at essentially negligible degradation in beam emittance.

Ultrabright and Ultrafast III–V Semiconductor Photocathodes

Crucial photoemission properties of layered III-V semiconductor cathodes are predicted using Monte Carlo simulations. Using this modeling, a layered GaAs structure is designed to reduce simultaneously the transverse energy and response time of the emitted electrons. This structure, grown by molecular beam epitaxy and activated to negative electron affinity, is characterized. The measured values of quantum efficiency and transverse energy are found to agree well with the simulations. Such advanced layered structures will allow generation of short electron bunches from photoinjectors with superior beam brightness.

Tip-geometry enhanced cooling of field emission from the n-type semiconductor

The cooling effect of field emission from an n-type semiconductor was theoretically investigated in quest for a solid state cooler. The vacuum potential was exactly expressed in terms of the semiconductor cathode geometry. This leaded to the more accurate configuration-dependent calculations of the energy exchange and the cooling power. It has been shown that a sharper tip of semiconductor can yield either a larger field emission current density or a larger energy exchange, according to the applied bias. For an atomic size tip, the n-Si cathode yielded the cooling power density Γ = 2.0, 75, and 713 W/cm2 at temperature T = 300, 600, and 900 K, respectively. This implies that an optimized configuration of an n-Si cathode produces a significant electron emission cooling, especially at high temperatures.

Physical mechanisms of self-organization and formation of current patterns in gas discharges of the Townsend and glow types

The paper discusses current filamentation and formation of current structures (in particular, hexagonal current patterns) in discharges of the Townsend and glow types. The aim of the paper, which is in part a review, is to reveal basic reasons for formation of current patterns in different cases, namely, in dielectric barrier discharge, discharge with semiconductor cathode, and micro-discharge between metallic electrodes. Pursuing this goal, we give a very brief review of observations and discuss only those theoretical, computational, and experimental papers that shed light on the physical mechanisms involved. The mechanisms are under weak currents—the thermal expansion of the gas as a result of Joule heating; under enhanced currents—the electric field and ionization rate redistribution induced by space charge. Both mechanisms lead to instability of the homogeneous discharges. In addition, we present new results of numerical simulations of observed short-living current filaments which are chaotic in space and time.

Loss mechanisms and back surface field effect in photon enhanced thermionic emission converters

Photon Enhanced Thermionic Emission (PETE) solar converters are based on emission of energetic electrons from a semiconductor cathode that is illuminated and heated with solar radiation. By using a semiconductor cathode, photo generated electrons enable high electron emission at temperatures much lower than the common range for thermionic emitters. Simple models show that PETE conversion can theoretically reach high efficiency, for example, above 40% at concentration of 1000 suns. In this work, we present a detailed one-dimensional model of PETE conversion, accounting for recombination mechanisms, surface effects, and spatial distribution of potential and carrier concentration. As in the previous PETE models, negative space charge effects, photon recycling, and temperature gradients are not considered. The conversion efficiency was calculated for Si and GaAs based cathodes under a wide range of operating conditions. The calculated efficiencies are lower than predictions of previous zero-dimensional models. We analyze the loss mechanisms and show that electron recombination at the cathode contact is a significant loss. An electron-blocking junction at the cathode back contact is therefore essential for achieving high efficiency. The predicted efficiencies for Si and GaAs cathodes with homo-junction back surface field layers are both around 31%, but with more favorable assumptions on the contact structure, it may be near 40%. The analysis leads to important conclusions regarding the selection of cathode material and back surface junction configuration.

Surface behavior based on ion-induced secondary electron emission from semi-insulating materials in breakdown evolution

This study focuses on analyses of secondary electron emission (SEE) at semiconductor surfaces when the sufficient conditions of space–time distribution occur. Experimental measurements and calculations with the approach of Townsend coefficients, which include the evaluations of ionization coefficient (α) and SEE coefficient (γ) were performed in high-ohmic InP, GaAs, and Si semiconductor cathodes with argon and air environments in a wide range of E/N (300–10 000 Td). The direct calculations of γ were carried out to determine the behavior of cold-semiconductor cathode current in a wide range of microgaps (45–525 μm). Paschen curves are interpreted in the dependence of large pd range on breakdown voltage through γ and α/N. Ion-induced secondary electrons exhibit the direct behaviors affecting the timescale of breakdown evolution in the vicinity of the Paschen minimum during the natural bombardment process with ions of semiconductor cathodes. Also, when α/N rapidly drops and the excitations of gas atoms densely occupy the gas volume, we determined that the photoelectric effect provides a growth for electron emission from semiconductor surfaces at the breakdown stage at the reduced values of E/N. At all pressures, the emission magnitudes of electrons liberated by semiconductor cathodes into vacuum are found as γInP > γGaAs > γSi in breakdown evolution.

Hydrogen discharges operating at atmospheric pressure in a semiconductor gas discharge system

Analyses of physical processes which initiate electrical breakdown and spatial stabilization of current and control it with a photosensitive cathode in a semiconductor gas discharge system (SGDS) are carried out in a wide pressure range up to atmospheric pressure p, interelectrode distance d and diameter D of the electrode areas of the semiconductor cathode. The study compares the breakdown and stability curves of the gas discharge in the planar SGDS where the discharge gap is filled with hydrogen and air in two cases. The impact of the ionizing component of the discharge plasma on the control of the stable operation of the planar SGDS is also investigated at atmospheric pressure. The loss of stability is primarily due to modification of the semiconductor-cathode properties on the interaction with low-energy hydrogen ions and the formation of a space charge of positive ions in the discharge gap which changes the discharge from Townsend to glow type. The experimental results show that the discharge current in H2 is more stable than in air. The breakdown voltages are measured for H2 and air with parallel-plane electrodes, for pressures between 28 and 760 Torr. The effective secondary electron emission (SEE) coefficient is then determined from the breakdown voltage results and compared with the experimental results. The influence of the SEE coefficient is stated in terms of the differences between the experimental breakdown law.

Field effect controlled lateral field emission triode

Lateral field emission transistors show promise for many high frequency and high power applications. Typical lateral devices place a gate roughly in between the cathode tip and the anode. While effective, such devices require large gate voltages for device control. This study proposes relocating the gate on top of the semiconductor cathode stem, behind its emitting tip, allowing field effect transistor based control of the transistor. Both enhancement and depletion mode are possible, and the gate bias range needed for control becomes an easily designed parameter. Example structures are modeled where this range is about 1 V. Relocation of the gate has the additional benefit of simplifying the region between the anode and the cathode tip, thus opening up the possibility of shrinking their spacing.

Hexagonal structures of current in a “semiconductor-gas-discharge gap” system

The results of experimental investigation of the stability boundary for a spatially homogeneous state of a discharge in the planar gap of a “semiconductor-gas-discharge” cryogenic system filled with nitrogen are considered. The semiconductor cathode was prepared from single-crystalline silicon doped with a deep-lying impurity. Quantitative data are obtained for the conditions of formation of a hexagonal dissipative structure in the current distribution for two values of the discharge gap length upon a change in the gas pressure and in the conductivity of the cathode. It is found that for a fixed gap length, the formation of the critical state can be described approximately by a universal function of the electrode conductivity and gas pressure.

Photon-enhanced thermionic emission for solar concentrator systems

Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

A simple physical model of hexagonal patterns in a Townsend discharge with a semiconductor cathode

This paper explains the observed effect of self-organization in a dc driven planar gas discharge–semiconductor system resulting in a hexagonal current pattern under cryogenic conditions. It is shown that the electric field redistribution usually causing a falling current–voltage characteristic (CVC) of the Townsend discharge and the discharge instability cannot provide the formation of the hexagonal pattern. Another mechanism is proposed which gives a necessary, high negative slope of the CVC under cryogenic conditions. This is a well-known thermal mechanism. Due to Joule heat release gas is heated and expands; hence, a lower field and voltage are required to sustain the discharge at a given current. Simple approximate equations describing non-stationary spatially inhomogeneous states in the gas discharge–semiconductor system are derived from physical considerations. The numerical integration of the obtained equations with a realistic parameter set gives the hexagonal current pattern. By simplifying these equations, we found analytically the current and the discharge voltage distributions of the hexagonal type and a simple formula for the distance between adjacent current filaments. The analytical solution allows one to investigate the roles of different factors and extract from experiment the negative differential resistance of the discharge, which is the main parameter in the problem of the discharge instability and the current structure formation.

Theoretical analysis of the energy exchange and cooling in field emission from the conduction band of the n-type semiconductor

Field emission has been theoretically found to contribute to the cooling only for the semiconductor cathodes. Using the formal theory developed recently by authors, the authors have calculated the energy exchange Δε as a function of temperature T and field F. It is found that the obtained Δε is positive for all T and large enough for a considerable cooling at room temperature. Even when the Joule heating is considered, field emission yields the net cooling effect. It is also found that the cooling is more effective for the n-GaN cathode than for the n-Si.

Memory effect in semiconductor gas discharge electronic devices

The memory effect in the planar semiconductor gas discharge system at different pressures (15–760 Torr) and interelectrode distances (60–445 µm) was experimentally studied. The study was performed on the basis of current–voltage characteristic (CVC) measurements with a time lag of several hours of afterglow periods. The influence of the active space charge remaining from the previous discharge on the breakdown voltage (UB) has been analysed using the CVC method for different conductivities of semiconductor GaAs photocathode. CVC showed that even a measurement taken 96 h after the first breakdown was influenced by accumulated active particles deposited from the previous discharge. Such phenomena based on metastable atoms surviving from the previous discharge and recombined on the cathode to create initial electrons in the avalanche mechanism are shown to be fully consistent with CVC data for both pre-breakdown and post-breakdown regions. However, in the post-breakdown region pronounced negative differential conductivity was observed. Such nonlinear electrical property of GaAs is attributed to the existence of deep electronic defect called EL2 in the semiconductor cathode material. On the other hand, the CVC data for subsequent dates present a correlation of memory effect and hysteresis behaviour. The explanation for such a relation is based on the influence of long lived active charges on the electronic transport mechanism of semiconductor material.

Stability and current behaviour in semiconductor gas discharge electronic devices

Breakdown and range of stable discharge glow in a homogeneous dc electric field are studied at various distances d between the electrodes and different inner diameters D (5, 9, 12, 18 and 22 mm) of GaAs semiconductor cathode areas. The current–voltage characteristics of the gas discharge system have been studied in a wide range of pressure p (16–760 Torr), interelectrode distances d (10 µm–5 mm) and conductivities of the GaAs cathode. The initiation of electrical breakdown as a result of secondary electron emission from the semiconductor cathode in low gas pressure is presented in this paper. In a planar gas discharge cell with diameters much larger than an interelectrode distance, the effects of different parameters (overvoltage, electrode separation, diameter and conductivities of the GaAs cathode, gas pressure, glow current, etc) on electrical breakdown and spatial stabilization of the current have been studied. The distributed resistance of photosensitive semiconductor cathode and the impact of the ionizing component of the discharge plasma on the control of the stable operation of a planar gas discharge system at atmospheric pressure are also investigated. Through spatially uniform irradiation of the semiconductor cathode, non-stationary states which are non-homogeneous can be generated in a system. The loss of stability is primarily due to the formation of a space charge of positive ions in the discharge gap which changes the discharge from the Townsend to the glow type.

Energy exchange in field emission from semiconductors

Field emission from semiconductors has been theoretically investigated by describing the energy transfer of electron. The authors developed a theory for the replacement process to calculate the energy exchange Δε between the electrons outgoing from and incoming to the semiconductor cathode. The obtained Δε is found to increase with increasing T and decreasing field F but to be almost independent of doping. It is also found that Δε is always positive, implying the cooling effect of field emission from the semiconductors for all temperatures.

Universal characteristics of resonant-tunneling field emission from nanostructured surfaces

We have performed theoretical and experimental studies of field emission from nanostructured semiconductor cathodes. Resonant tunneling through electric-field-induced interface bound states is found to strongly affect the field-emission characteristics. Our analytical theory predicts power-law and Lorentzian-shaped current–voltage curves for resonant-tunneling field emission from three-dimensional substrates and two-dimensional accumulation layers, respectively. These predicted line shapes are observed in field emission characteristics from self-assembled silicon nanostructures. A simple model describes formation of an accumulation layer and of the resonant level in these systems.

In situcleaning of metal cathodes using a hydrogen ion beam

Metal photocathodes are commonly used in high-field rf guns because they are robust, straightforward to implement, and tolerate relatively poor vacuum compared to semiconductor cathodes. However, these cathodes have low quantum efficiency (QE) even at UV wavelengths, and still require some form of cleaning after installation in the gun. A commonly used process for improving the QE is laser cleaning. In this technique the UV-drive laser is focused to a small diameter close to the metal's damage threshold and then moved across the surface to remove contaminants. This method does improve the QE, but can produce nonuniform emission and potentially damage the cathode. Ideally, an alternative process which produces an atomically clean, but unaltered, surface is needed. In this paper we explore using a hydrogen ion (H-ion) beam to clean a copper cathode. We describe QE measurements over the wavelength range of interest as a function of integrated exposure to an H-ion beam. We also describe the data analysis to obtain the work function and derive a formula of the QE for metal cathodes. Our measured work function for the cleaned sample is in good agreement with published values, and the theoretical QE as a function of photon wavelength is in excellent agreement with the cleaned copper experimental results. Finally, we propose an in situ installation of an H-ion gun compatible with existing s-band rf guns.