Vacuum Interface

Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 1141−1149

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Optically Driven Tunable Transistor Effect at Matter/Vacuum InterfaceToward Dielectric Optical Transistors Mikolaj Lukaszewicz,† Bartlomiej Cichy,*,† Dominika Wawrzynczyk,‡ and Wieslaw Strek*,† †

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Polish Academy of Sciences, Institute of Low Temperature and Structure Research, Department of Optical Spectroscopy, Okólna 2, 50-422 Wrocław, Poland ‡ Wroclaw University of Science and Technology, Faculty of Chemistry, Advanced Materials Engineering and Modelling Group, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland S Supporting Information *

ABSTRACT: The laser controlled transistor effect at the dielectric/vacuum interface characterized by tunable current− voltage characteristics was studied in this work. An Yb3Al5O12 nanoceramic was used as the dielectric substrate. It was shown that continuous wave near-infrared (975 nm) laser excitation is able to induce effective charge accumulation in the Yb3Al5O12/vacuum interface region. The accumulated charge may further be freely transported in the vacuum channel between the electrodes. The observed effect was discussed in terms of electric field enhanced thermionic electrons emission from nanoscale objects. Such electron release from nanoscale objects is seen to be strictly related to the nanoparticle’s surface, including exposure of a number of broken bonds which are associated with a large charge state that cannot be easily compensated under vacuum conditions. Performed experiments revealed that the laser-induced charge accumulation effect is a nonlinear process characterized by a threshold point of ca. 0.8 W/mm2 for λ = 975 nm of optical density. The investigated device was characterized by a very large relative current gain (105) and tunable output characteristics. It was also shown that the output characteristic for such a dielectric transistor may be freely tuned from fully symmetric to asymmetric blocking, which is characteristic for rectification behavior of the classical p−n junction. Due to the dielectric properties, free space propagation of the gate signal, and high mobility of nearly free electrons, this kind of transistor may be seen to be used for special applications involving hazardous environments, very fast switching, vacuum conditions (cosmic space), or high voltage switching where galvanic isolation of the gate electrode is of high importance. KEYWORDS: vacuum transistor, dielectric transistor, tunable transistor, phototransistor, white light, electrons emission

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INTRODUCTION Tremendous development of integrated circuit (IC) based electronics started, in fact, at Bell Laboratories in 1948 with the invention of the point-contact transistor. A piece of single germanium crystal opened the way for engineered electron flow control in a solid-state phase and initiated a new era in the construction of electronic devices. Successful discovery of the transistor effect at that time was possible thanks to the two following main factors: (i) Shockley’s prediction about the possibility of intentional control of electron flow in a semiconductor crystal by application of an external electric field and (ii) Bardeen’s contribution to the theory of semiconductors’ surface physics regarding the impact of surface states on rectification at the metal−semiconductor contact.1,2 This brilliant concept of intentionally controlled electron flow in solids is still alive, evolving into many possible variants of the basic electron−electric interaction. Over the many years, this first point transistor gave rise to such realizations as bipolar transistors (BJT), field effect transistors (FET, MOSFET), bipolar transistors with an isolated gate (IGBT), etc.3 © 2019 American Chemical Society

Having a closer look at the simplest case of BJT transistor, electron flow control is usually realized at the boundary of differently doped semiconductors, i.e., p−n junction, behind which the formation of the interfacial potential barrier as well as the possibility of its modulation by externally applied electric field is responsible for the transistor effect. The presence of a boundary region at the p−n junction may therefore be considered as a key element in realization of all sorts of electron−electric field interaction alternatives. In a similar way, a boundary region with a potential barrier associated with the existence of a matter/vacuum interface resulted from limited periodicity of any nanoscale object placed in a vacuum may be obtained. Such an interface is inseparably connected with a number of broken bonds, carrying an uncompensated electrical charge, which as a consequence results in a potential energy barrier build-up. This in turn is responsible for extensive changes in the subinterfacial Received: March 11, 2019 Accepted: June 21, 2019 Published: June 21, 2019 1141

DOI: 10.1021/acsaelm.9b00150 ACS Appl. Electron. Mater. 2019, 1, 1141−1149

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ACS Applied Electronic Materials

Figure 1. Two variants of measurement setup used for electrical characterization of the YbAG ceramic. (a) Lateral configuration used for transistor effect investigation. (b) Vertical configuration used for analysis of photoinduced charge generation at the YbAG/vacuum interface. (c) Simplified band scheme for the Ag/YbAG/AG structure. TE and FE stand for thermal and field emission, respectively.

further investigated by a number of researchers, showing its anti-Stokes character that was in many ways quite similar for a broad range of various materials, including graphene, metals, or oxides.5−23 The WLE reveals a number of uncommon properties, i.e., (i) broad-band emission, (ii) independence on the excitation wavelength, (iii) threshold behavior of light emission, (iv) sensitivity to the vacuum level, (v) moderate average temperature (lower than ca. 500 °C) of the emitting sample, or (vi) multiphoton character. In this work, we have shown that WLE is inseparably related to efficient generation of electronic charge at the interface region, which may be easily thermalized giving rise to a broad-band emission of light. In this study, we exploit yet another interesting feature of WLE phenomena and showed that it may also be adopted for construction of phototransistors characterized by galvanic isolation between the gate and the channel. It is also shown that the proposed solution allows for dynamic modulation of I−V characteristics, which is a completely new feature that was not possible for realization in classical semiconductor-based transistors. The proposed solution fits in the general field of nanoscale vacuum channel transistors (NVCT) for which the carriers are transported through a vacuum. In the majority of realizations, such transistors are based on the field emission of cold electrons followed by their transport over a planar or vertical vacuum channel.24−26 Nirantar et al. have also recently presented an interesting metal-based, field emission air channel transistor.27 Speaking of the phototransistors, Srisonphan presented a vacuum field-effect phototransistor based on graphene/Si heterojunctions in association with the graphene/ SiO2/Si field effect structure.28 Another hybrid solution for a vacuum phototransistor was also presented by Konstantatos et al., who used graphene/quantum dots layers as the photoactive element.29 In this paper, we discuss a severely different solution for the NVCT. In our approach, the YbAG substrate representing the active medium does not exhibit any absorption band for the 975 nm laser beam. The electron release from the YbAG nanoparticles is provided directly by the size and electric field enhanced thermionic emission during the WLE phenomenon. The provided solution enables a cheap and simple technology for high gain dielectric NVCT.

region, e.g., inductive local redistribution of the electronic density in the subinterface region. The local change in the electronic density is responsible further for locally distinct charge mobility, change in the density of states (DOS), or type of conductivity. In the case of such limited crystal structure periodicity, the existence of an externally tunable interfacial potential energy barrier may be a fundamental feature that would allow one to control current flow in non-semiconducting solid crystals. In this work, we adopted the presented analogy proposing realization of the transistor effect at the dielectric/vacuum interface and a focused laser beam as the optical excitation. In its most basic form, the transistor is formed of a dielectric Yb3Al5O12 (YbAG) nanoceramic substrate on which two silver electrodes are deposited on both sides (Figure 1a). Laser excitation of the YbAG substrate placed in a vacuum leads typically to a strong charge induction at the dielectric/vacuum interface. The accumulated charge is seen to be released from the dielectric surface and may be freely moved in the vacuum channel between the electrodes, providing the transistor effect. It is important to note that in such an approach the YbAG substrate is just a reservoir of electrons and does not participate in the electrical carriers’ transport. The proposed device, with several restrictions, could then be discussed in some analogy to a field-effect transistor with an induced channel. All of the important features of the proposed transistor may be summed up as follows: (i) charge is extracted from dielectric nanoparticles by a focused laser beam, (ii) current flow is provided by the vacuum channel, and (iii) the transistor effect may be dynamically changed into a rectifying-like one by simple spatial relocation of the laser focal point. We have shown in this work that induction of an electrical channel at the dielectric/vacuum interface coexists with the phenomenon of laser-induced anti-Stokes white-light emission (WLE) from dielectrics. The WLE phenomenon of insulating nanophosphors was first observed and described by Wang and Tanner in 2010, who had studied an odd broadband emission from Yb2O3, Sm2O3, and CeO2 powders that occurred under intense 975 nm laser excitation.4 Due to its untypical character, the WLE was first seen as a new type of cold emission and 1142


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Figure 2. SEM images, compositional chemical analysis (EDS), and compositional mapping of (a) as prepared YbAG powders as well as (b) the YbAG ceramic’s surface. Photography of the YbAG ceramic along with its dimensions is shown as an inset in part b. (c) EDS mapping of the as prepared YbAG powder and (d) YbAG ceramic substrate obtained by high pressure-high temperature pressing.

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ceramic but is strictly related to the YbAG/vacuum interface without damage of the dielectric YbAG ceramic substrate. Irradiation of the YbAG ceramic allows therefore the investigation of the impact of laser excitation on transport properties in the light induced channel. A second setup of a vertical geometry and vertical vacuum gap between the anode and cathode (Figure 1b) was used for analysis of the surface charge state changes on the YbAG nanoceramic.32 In this configuration, the YbAG nanoceramic is placed on a glass substrate covered with a thin layer (100 nm) of indium−tin oxide (ITO), forming the cathode, and is irradiated by a free coupled laser beam. The anode in this case is realized as a thin glass substrate covered with a layer of transparent ITO. Any charge accumulation at the cathode must then be related to an electrical compensation effect at the anode, resulting in an electrical current flow in the circuit. All of the experiments were performed in a vacuum chamber equipped with transparent quartz windows. A near-infrared (NIR) continuous wave (CW) laser diode (LD) manufactured by CNI Optoelectronics Tech. Co., Ltd. was used as the excitation source. A commercial BK-7 biconvex lens was used for focusing the laser beam into a spot of 0.5 mm2 and a maximum optical density of 4.2 W/mm2 obtained for the highest laser diode power of 2.1 W. An AVS-USB2000 Avantes Spectrometer was used for the collection of a broadband emission spectrum accompanying the electron plasma build-up at the YbAG/vacuum interface. The WLE spectra were collected using a KG3 optical filter blocking the excitation wavelength. Spectra were corrected for detector efficiency

EXPERIMENTAL SECTION

Ytterbium aluminum garnet (Yb3Al5O12, YbAG) nanoparticles were prepared according to a modified Pechini method.30 Ytterbium oxide Yb3O3 (Stanford Materials Company, 99.998%), aluminum chloride AlCl (Alfa Aesar, 99.9995%), ultrapure HNO3, citric acid C6H8O7 (POCH, 99.5%), and ethylene glycol C2H6O2 (POCH, 99%) were used as received without further purification. First, the Yb(NO3)3·xH2O was prepared by dissolving Yb3O3 in ultrapure HNO3 acid. The ytterbium nitrate along with aluminum chloride were then dissolved in a mixture of aqueous citrate acid and ethylene glycol solution resulting in a molar ratio for Yb/Al/C6H8O7/ C2H6O2 of 3:5:50:20. The mixture was further stirred for 3 h and dried at 90 °C for 7 days in order to obtain a gel. The obtained resin was calcinated at a temperature of 1000 °C for 8 h, giving a final product. The YbAG ceramic was prepared by compressing YbAG powder using the hot isostatic pressing (HIP) sintering method31 at 800 °C and 8 GPa. Two different setups were used in the experiments (see Figure 1). The first one is the planar setup depicted in Figure 1a showing the schematic construction of the dielectric transistor. Two silver electrodes were formed in the lateral form at the edges of the YbAG nanoceramic. In this realization, the electrodes are fully symmetric, and each of them may take a role of the source (S) or the drain (D). Space between the electrodes defines the vacuum channel for ballistic flow of electrons. It is important to note that for such a configuration, the electric channel is not formed inside the YbAG 1143


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ACS Applied Electronic Materials and filter transmittance. All of the electrical measurements were performed using a Keithley 2410 Source Meter. Crystallographic characterization was performed with an X’Pert PRO PANalytical X-ray powder diffractometer equipped with a linear PIXcel detector and a Cu Kα radiation source. The structural data refinement was performed with MAUD software. The morphology and composition of YbAG powders and pressed ceramics were characterized with a JEOL JSM-6610LVnx scanning electron microscope (SEM) equipped with an Oxford Aztec Energy energy-dispersive X-ray spectroscopy attachment (EDS).

cooperative emission,16 up-conversion emission following multiphoton absorption,35 thermal radiation,15,36 photon avalanche,37 supercontinuum generation,38 or intervalence charge transfer (IVCT).8,39 The latter deserves closer attention, as formation of mixed valence pairs, i.e., Yb3+ and Yb2+, may not be ignored during intense pumping with the 975 nm CW laser in a vacuum. In such a case, the charge transfer process may occur either by optical or thermal excitation. Electrical Properties and Transistor Effect. When the transport properties of the WLE emitting YbAG nanoceramic are studied, distinct changes of up to 5 orders of magnitude in the sample’s conductivity may be noticed each time the WLE emission threshold point has been reached. Observed large relative changes in conductivity are in fact very surprising, as due to the wide band gap of YbAG of around 6−7 eV33,40,41 the electron promotion to the conduction band using low energy excitation photons seems to be controversial. This implicates that mechanisms responsible for an increase in the current flow must be related strictly to the surface and optical disturbance in the minimized surface charge state. This argumentation seems to be well justified considering large amounts of energy that are carried by the surface of any kind of nano- or submicrometer objects. An investigation of such surface charge state changes in the YbAG ceramic was then performed using the capacitive-like electrode configuration presented in Figure 1b. For such a case, the YbAG nanoceramic was placed on a glass/ITO substrate forming a glass/ITO/YbAG cladding-like cathode (see Figure 1b). The glass/ITO anode was then vertically positioned over the cathode, leaving a well-defined vacuum gap (150 μm) between the anode and cathode. An important feature of the capacitive configuration is that any changes related to accumulation of charge occurring at the YbAG ceramic surface must be instantaneously mirrored by anode charge flow giving rise to electrical current flow Ip in the circuit. Representative results of the experiments taken for three different external bias voltages, i.e., 0, 10, and 50 V, are shown in Figure 3. The 0 V (Figure

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RESULTS AND DISCUSSION Sample Characterization. The XRD diffraction pattern of as-prepared YbAG powder is shown in Figure 1S (in Supporting Information). Obtained reflections stay in good correspondence with theoretical predictions and a reference pattern (ICSD No. 170160).33 According to the results, the obtained powder was identified as Yb3Al5O12 crystallizing in a garnet structure (space group No. 230). No parasitic phases were identified in the analyzed powder. According to the Rietveld refinement analysis, the average crystal size was estimated as 57.6 nm, the obtained unit cell parameter was calculated as a = 11.9568 Å, and the microstrain was established at 0.00188. Representative SEM images of the as synthesized YbAG powder and YbAG ceramic’s surface have been presented in Figure 2. According to the results, one may notice significant aggregation of YbAG particles into bigger grains exceeding 100 μm (Figure 2a). This seems to be well rationalized taking into account the quite high sintering temperature. The EDS analysis revealed very good correlation between the experimentally measured chemical composition of the samples and assumed stoichiometry (Figure 2c). The YbAG ceramic revealed both nanoscale crystalline internal structure and macroscopic dimensionality formed as a densely packed solid pellet (inset in Figure 2b), which was preferential due to its usability in electrical characterization and well-defined localization of laser excitation. Optical characterization of the anti-Stokes WLE emission from the YbAG ceramic was performed using focused 975 nm laser excitation. During the experiments, the YbAG nanoceramic was kept in a vacuum chamber at reduced pressure (10−4 hPa). Representative emission spectra of the WLE taken as a function of the optical excitation power are shown in Figure 2Sa. After crossing the threshold point, the photoluminescence PL intensity of the WLE increases nonlinearly in accordance with the power law formula,34 showing the process order parameter of N = 4.34 (Figure 2Sb) and suggesting that the WLE originates from multiphoton excitation. This seems to be unlikely because the probability of simultaneous absorption of four or more photons in a single quantum mechanical act seems to be very low, especially if one considers the range of excitation powers and photon fluxes that were used in the experiment. It has been also shown in several former reports9,13,22 that the character of the WLE emission is almost independent of the material. In the most straightforward way, the blackbody radiation theorem seems to be a mechanism of choice for interpretation of the WLE emission. There are, however, several factors that hinder this interpretation (i.e., its threshold behavior, moderate temperature of emitting sample (ca. 500 °C), and saturation of the WLE intensity for higher excitation powers), thus several different theories have already been proposed for interpretation of the WLE phenomenon. Those propositions include

Figure 3. Photocurrent generation under IR laser excitation. Laser power dependence of the current under no voltage bias (a), 10 V (b), and 50 V (c) of bias voltage. Characteristics were taken for the vertical configuration for which the anode and cathode were spaced by 150 μm.

3a) case is of special interest as it provides information needed for better understanding of the light−YbAG interaction. When analyzing this characteristic, one may notice a strongly nonlinear increase of the Ip current as a function of the optical excitation power density. Clear correspondence to the results presented in Figure 2b is also noticed. Both characteristics depict an evident threshold point located at almost this same optical power level. Crossing the threshold point, one observes an increase in the Ip current value as well as initiation of the WLE emission. Application of an acceleration potential, i.e., 10 1144


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Figure 4. Current gain in YbAG under 975 nm excitation: (a) in a series of laser diode power as a function of bias voltage with an inset showing the characteristics in the 0−10 V range and (b) optical power density dependence of current gain at 100 V. Characteristics were taken for planar configuration. The distance between anode and cathode was chosen to 1 mm.

Observed changes in the conductivity may now be clearly associated with the surface charge accumulation and its vacuum channel transport in the external bias voltage. In its simplest realization, such a transistor may be discussed in analogy to a field-effect transistor with an enhanced channel. The focused laser beam resembles a transistor’s gate electrode. Accumulation of electrons at the YbAG/vacuum interface brings an analogy to the enhanced channel field-effect transistors. Spatial localization of the electrons brings a very valuable feature that will be described later in the text. For this realization, the electric channel is defined by two lateral electrodes, which may serve symmetrically (depending on convention) as the S and D electrodes, as has been depicted in Figure 1a. The representative transfer and output characteristics of such transistor are shown in Figure 4. According to the output characteristic (Figure 4a), one may notice that the drain current ID gain reaches up to 5 orders of magnitude for the highest value of optical power density (4.2 W/mm2). Distinct saturation of the ID current is typically observed for voltages over 20 V. It seems to be quite clear that for other materials the overall charge generated by the laser beam may be of different value and the saturation level may also be changed. The transfer characteristic has been shown in Figure 4b. One may see that below the threshold point almost no ID charge accumulation and current flow is noticed. Crossing the threshold point of ca. 0.8 W/mm2, the ID starts to increase nonlinearly. No evident saturation of the ID current flow was observed up to 4.2 W/mm2 of laser excitation. However, it is clear that electron release must be at least self-limiting due to the fact that a large amount of electronic charge must be related to strong screening of the optical field, diminishing the overall interaction of electromagnetic field with the sample. An interesting feature of the results is the significantly higher value of the process order parameter NEL = 8.24, as compared to the value obtained from the optical experiments NPL = 4.34 (Figure 2Sb), with the former being nearly twice as large as the latter. The difference may be related to the fact that each electron generated gives its impact to the overall current flow and not each photon emitted by the electrons is detected by the CCD camera. Another interesting feature of the phenomenon is the strong spatial confinement of the electron cloud and additional

V (Figure 3b) and 50 V (Figure 3c), results in a dramatic increase of the Ip current from 1.2 nA for 4 W/mm2 up to 20 nA and 80 nA, respectively. This laser dependent generation of charge at the YbAG/vacuum interface has very important technological implications, providing a novel concept for the transistor effect in a dielectric medium. This concept goes beyond conventional field mediated current flow control in a semiconductor, providing direct light management over charge accumulation and flow, as well as dynamically modulated output characteristics. A number of unique properties can emerge for such a transistor, i.e., (i) no current flow is possible for an unexcited sample, (ii) maximum voltage between source and drain electrodes is equal to the breakdown voltage of the dielectric medium, (iii) no ambient light or sunlight is able to work as a gate signal, (iv) output characteristic of the transistor may be simply modulated, (v) there are no limitations for the excitation laser wavelength, (vi) electrons moving in a vacuum have large mobility, and (vii) there is a galvanic isolation between the gate and source/drain circuits. These results bring, aside from their large technological implications, several important pieces of information about the WLE mechanism itself. First, a strong correlation between the Ip and WLE implies that WLE is generated by a free or nearly free electron cloud accumulated at the YbAG/vacuum interface. Taking into account the spot-like character of WLE, it is also justified that the electron cloud is strongly confined to the region of laser excitation. The nearly free electrons related to non-negligible interaction with the surface potential barrier suggest that the process must lead to a dynamic equilibrium between the electronic recombination and generation pathways. The existence of free or nearly free electron accumulation at the YbAG/vacuum interface is also related to the existence of a wide spectrum of energy states. It is likely that such a wide spectrum of states may be responsible for the broadband emission of light being easily thermalized by 975 nm excitation laser. The presence of such an electronic cloud brings also new light to the large values of the process order parameter N, as it is well-known that many photons may be absorbed by electronic plasma. Similarly to the process order parameter N, the threshold point may then be easily rationalized as a point at which electron concentration is large enough to give noticeable light emission by effective laser thermalization. 1145


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Figure 5. Representative output characteristics for two extreme points of excitation: (a) fully symmetric for middle point excitation and (b) asymmetric blocking characteristic for diode-like behavior. The characteristics may be smoothly modulated changing position of the excitation beam. Characteristics were taken for planar configuration. Distance between anode and cathode was chosen to 1 mm.

In the case of the investigated material and the used excitation source, the difference between the energy required for electron escape and 975 nm infrared photon energy (1.27 eV) is too high to justify the occurrence of a pure photoelectric effect, unless, unlikely in the range of used optical power densities, high-order multiphoton processes would be involved. Instead, one should approach the solution from the starting point of the model for thermionic emission,42 as described in eq 1

properties resulting from it, i.e., dynamical modulation of the output characteristic depending on the spatial position of the excitation laser beam. Representative characteristics for two extreme cases are shown in Figure 5. In the first case (Figure 5a), the YbAG ceramic was excited by a laser beam centered between the cathode and anode electrodes. According to the expectations, the I−V characteristics represent an almost fully symmetric character. When changing the excitation spot position toward the anode, one is able to antisymmetrize the current response. An example of such a kind of excitation is shown in Figure 5b. One may clearly see that for positive values of the anode−cathode voltages VAC, the device behaves as a typical transistor, but for reversed polarization, a diode-like blocking behavior is well noticed. An interesting feature of that is the fact that in contrast to semiconductor diodes, no such thing as a breakdown reverse voltage Vbr is expected. A noticeable increase in the anode−cathode current is observed when increasing the reverse polarization voltage, which is well rationalized taking into account the values of reverse voltage of ca. −100 V. The blocking behavior shown in Figure 5b seems to be a direct consequence of electrical repulsion of electrons from the negative electrode. As a result of that, all values of external bias voltage that are below the dielectric or vacuum breakdown voltage are allowed for operation of the transistor. We believe that a deeper understanding of the mechanisms that are responsible for charge accumulation at the YbAG/vacuum interface may be helpful for optimization of any important parameters of the described electrical device. Modeling and Theoretical Predictions. As works on the work function of wide band gap materials are not common in the literature, for the needs of subsequent calculations, the value of energy between the vacuum level and the top of the valence band is assumed as the energy required to free an electron from the system, which in the case of YAG-based materials is around 9.5 eV. In order to explain the obtained results, a comparison with known models of electron emission is drawn. While the internal photoconductivity in Yb-doped YAG could be explained with models such as the hopping electron conductivity or nonradiative branch of the IVCT, enhanced with a temperature-related increase of carrier mobility, this information is insufficient for the description of charge carriers leaving the material surface.

i −W zy zz J = AG × T 2 × expjjj k kT {

(1)

where J is the current density, AG is a parameter, T is the temperature, W is the work function of the material, and k is the Boltzmann constant. This model seems to be a good starting point considering the fact that local temperature in the focal point of laser excitation may be way higher than the average temperature of a whole sample. Intense interaction of the electromagnetic field of optical frequency requires one to provide some extensions to the model. The field enhancement of electron emission is here discussed within the model of the Schottky effect. The electrical field applied in the direction of expected electron emission lowers the effective value of energy required to emit the charge carriers from the material by the value ΔW, presented in the equation below with lower index S to signify that it is related to the Schottky effect:43 i −(W − ΔWS) yz zz J = AG × T 2 × expjjjj z kT k {

(2)

The factor ΔWS reducing the energy required in the electron emission process is defined as ΔWS =

e 3F 4πε0

(3)

where e is the electron charge, F is the magnitude of the electrical field, and ε0 is the electrical permittivity of a vacuum. In the experiment, the distance between the two parallel electrodes is equal to 150 μm, which yields ΔWS values of 0.01 and 0.023 eV for 10 and 50 V of applied voltage, respectively. However, when calculating the theoretical values of current density resulting from field enhanced thermionic emission from eq 2 and taking into account the affected area of the 1146


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Figure 6. Current curves obtained from theoretical calculations of current density of emitted electrons as a function of temperature. Calculation results are presented in (a) semilogarithmic scale in a wide range of current temperatures and (b) in a close-up between 2200 K and 2500 K. Linear scale was used to maintain correspondence to the values obtained from the measurements.

and may be sufficient for the description of the phenomenon, at least at the current level of knowledge on the topic. One factor that differs strongly between the theoretical approach and the experiment is the stronger influence of electrical field observed in the latter. This can be related to the simplification of the model, in which only the area of the laser spot is taken into account, while in reality a bigger area on the material surface can be affected with laser-induced heating, increasing the emitted electron current by an additional fieldenhanced thermionic emission which can occur in its direct vicinity. In any case, a significant amount of emitted electron current can be measured in a setup based on optically active insulating material under relatively low optical excitation power, which is additionally sensitive to the magnitude of applied electrical field, expanding its area of applications.

material, the resulting values of electrical current are a few orders of magnitude smaller than the measured values when only a reasonable temperature range (as no material damage was observed after the measurements) is taken into consideration. This discrepancy implies that the role of the optical excitation is not limited to a source of heat, even if the photon energy is as small as in this case, and the obtained results cannot be explained only by the effects of locally increased temperature and the electrical field, and the energy of the absorbed photons should be included as well. The phenomenon known as the photon-enhanced thermionic emission (PETE) is well documented in the literature; however, as with previously mentioned models, its description is focused mostly on semiconductors with relatively narrow band gaps, thoroughly investigated in the aspect of their electronic properties, such as electron affinity, strongly related also to the morphology of the material, requiring thorough measurements. As such, the model established for the more advanced experiments on PETE in semiconductors44 is not used in this work, opting for a much simpler approximation instead. As such, it is proposed that the energy supplied by the incident photons further lowers the energy required for the emission of an electron into the vacuum: ij −(W − ΔWS − Eph) yz zz J = AG × T 2 × expjjjj zz kT k {

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CONCLUSIONS AND OUTLOOK Comprehensive studies on a new type transistor effect have been shown in this paper. It was shown that it is possible to optically generate and control the electrical charge in a strictly dielectric medium, such as the ytterbium−aluminum garnet (YbAG) ceramic. The results of the investigation of the phenomenon occurring under focused 975 nm laser diode beam excitation and the characteristics obtained therein allow for several promising aspects of proposed phototransistor devices to be derived in terms of diversity of their application. Due to strong charge accumulation at the matter/vacuum interface under laser excitation, high charge mobility, and an extremely low value of dark current, efficient amplification and fast switching of electrical current in the circuit can be realized. Furthermore, due to the initial low value of the current, and by extension the low conductivity of the unilluminated active material, there is a drastic difference between the absolute and relative optical responsivity of the transistor. The first is defined by the ratio between the observed photoinduced current increase under excitation and the optical power absorbed by the material, IPC/ Pexc, expressed in A/W. The latter is defined by the quotient IPC/Idark and is a dimensionless measure of current gain achievable under given conditions. While the absolute responsivity in the described material yields only 5 × 10−5 A/W, significantly lower than in the case of organic- or semiconductor-based phototransistors,45−50 the relative re-

(4)

where Eph is the photon energy, 1.27 eV in the case of 975 nm excitation. The results of theoretical calculations are presented in Figure 6, including the curves obtained with and without the inclusion of photon energy in the equation. The results are presented in values of electrical current, obtained by multiplying the current density J by the area of the focused laser beam spot. Even though the model is simple and may require refinement in the future, along with a broader investigation of electronic properties of studied dielectric materials, the calculated values fall within the range of what was observed in the experiment. It is possible that the energy equivalent of the work function could be estimated with better accuracy and would yield a smaller value, as the expected values of current still lie in a temperature range above the melting point of YbAG. However, the achieved result is not unlikely in this form 1147


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ACS Applied Electronic Materials sponsivity can reach as high as 105 times the current under 4 W/mm2 of optical excitation power density, resulting in a device in which the current is efficiently changed by several orders of magnitude, at the same time remaining in the low range of its absolute values. Furthermore, the process of amplifying and returning to the original value of the passing current is also fully reversible, as shown in the temporal characteristic of photoinduced current (Figure 3S). In contrast to narrow-gap semiconductors used in photodiodes and phototransistors,51 the optically active material is an electric insulator. As such its dielectric strength is naturally much higher, and no additional insulating material needs to be considered in order to avoid breakdown. No voltage breakdown was observed in preliminary estimations in the full measurement range of 500 V applied over a distance of 1 mm of the material (Figure 4S), translating to a value of dielectric strength of at least 500 kV/m. Another beneficial trait resulting from selecting a dielectric as the optically active material is its relation between temperature and electrical conductivity. As it is enhanced along with the increasing temperature, the working temperature of the device is only limited by the usable temperature range of other electrical components, such as the wiring or the electrodes, as in most cases it is far exceeded by the melting point of lanthanidedoped crystalline oxides, the only limiting factor in thermal durability of the optically active material. Due to the scalable method of synthesis and reagent costs of insulating nanophosphor fabrication, the production of photoactive material can be successfully performed on a large scale and is costefficient. Owing to the vast knowledge and fast development of the science of rare-earth-based phosphors, the process can be freely modified in order to achieve refinement of the desired material parameters within a given group of materials or a new branch of materials obtained altogether, either by controlling the dopant concentration, changing the species of dopant ions, or switching to a different crystalline matrix, all leading to a great potential in customizing the perspective properties of the device. Furthermore, the magnitude and proportion of current flow in opposite directions is sensitive to the position of the laser beam spot relative to the electrodes, allowing for consideration of motion detection or alternative means of current flow control, manipulating the beam spot position instead of excitation power. In terms of photocurrent generation, the absolute values are again small in magnitude, but the process of electron emission is efficient enough to consider its application in the alternative setup, containing a vacuum gap between the nanocrystalline material surface and the collecting electrode, expanding the concept of switching between the dark nonconducting and photo-excited conducting state of the device even further.

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electrical conductivity and current voltage characteristics of the YbAG ceramic taken between −500 and 500 V (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bartlomiej Cichy: 0000-0001-9166-2728 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge R. Tomala for help in preparation of the nanoceramics. W.S. would like to acknowledge partial support from the OPUS 15 (2018/29/B/ST5/ 00819) grant founded by the National Science Centre Poland.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00150. X-ray diffraction pattern taken for the Yb3Al5O12 powder, photoluminescence emission spectra from the YbAG ceramic under 975 nm excitation shown as a series of laser power, laser power dependence of integrated emission intensity for 975 nm excitation, optical power dependent dynamic characteristics of electrons’ emissivity described by overall change in 1148


Article

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