Showing papers on "Depletion region published in 2011"
TL;DR: The selective detection of two different gases, NO(2) and C( 2)H(5)OH, has been achieved using a p-type Co(3)O(4)-decorated n-type ZnO nanowire (NW) network sensor.
272 citations
TL;DR: Through the use of vertical junctions in resonant modulators, the lowest power consumption, lowest voltage, and smallest silicon modulators demonstrated to date are achieved.
Abstract: Vertical junction resonant microdisk modulators and switches have been demonstrated with exceptionally low power consumption, low-voltage operation, high-speed, and compact size. This paper reviews the progress of vertical junction microdisk modulators, provides detailed design data, and compares vertical junction performance to lateral junction performance. The use of a vertical junction maximizes the overlap of the depletion region with the optical mode thereby minimizing both the drive voltage and power consumption of a depletion-mode modulator. Further, the vertical junction enables contact to be made from the interior of the resonator and therein a hard outer wall to be formed that minimizes radiation in small diameter resonators, further reducing the capacitance and drive power of the modulator. Initial simple vertical junction modulators using depletion-mode operation demonstrated the first sub-100fJ/bit silicon modulators. With more intricate doping schemes and through the use of AC-coupled drive signals, 3.5μm diameter vertical junction microdisk modulators have recently achieved a communications efficiency of 3fJ/bit, making these modulators the smallest and lowest power modulators demonstrated to date, in any material system. Additionally, the demonstration was performed at 12.5Gb/s, required a peak-to-peak signal level of only 1V, and achieved bit-error-rates below 10−12 without requiring signal pre-emphasis. As an additional benefit to the use of interior contacts, higher-order active filters can be constructed from multiple vertical-junction modulators without interference of the electrodes. Doing so, we demonstrated second-order active high-speed bandpass switches with ~2.5ns switching speeds, and power penalties of only 0.4dB. Through the use of vertical junctions in resonant modulators, we have achieved the lowest power consumption, lowest voltage, and smallest silicon modulators demonstrated to date.
216 citations
TL;DR: In this paper, the effect of oxygen on the degradation of inverted bulk heterojunction solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends has been investigated by monitoring current-voltage (jV)-curves, impedance spectra and charge extraction by linearly increasing voltage (CELIV) traces during the degradation process.
186 citations
TL;DR: Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates and only charge carriers generated in the depletion region were found to contribute to the photocurrent.
Abstract: Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)6]3-/4- electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ∼20 nm. Mott−Schottky analysis of differential capacitance measurements indicated a depletion region of ∼17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.
182 citations
TL;DR: In this article, nonvolatile bipolar resistive switching has been observed in an Au/BiFeO3/Pt structure, where a Schottky contact and a quasi-Ohmic contact were formed at the Au/BioO3 and BiFeO 3/Pte interfaces, respectively.
Abstract: Nonvolatile bipolar resistive switching has been observed in an Au/BiFeO3/Pt structure, where a Schottky contact and a quasi-Ohmic contact were formed at the Au/BiFeO3 and BiFeO3/Pt interface, respectively. By changing the polarity of the external voltage, the Au/BiFeO3/Pt is switched between two stable resistance states without an electroforming process. The resistance ratio is larger than two orders of magnitude. The resistive switching is understood by the electric field - induced carriers trapping and detrapping, which changes the depletion layer thickness at the Au/BiFeO3 interface.
108 citations
TL;DR: In this article, single crystalline vertically aligned P-type Cu 2 O nanowires are deposited on a silicon substrate using solid-vapor technique, without using a catalyst or pre-deposited buffer layers.
97 citations
TL;DR: In this paper, a double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors.
Abstract: CaCu3Ti4O12 (CCTO) ceramicsdoped with rare earth (RE) oxides, including Y2O3, La2O3, Eu2O3, and Gd2O3, were prepared by the traditional solid–state reaction method in order to investigate the effect of RE oxide dopants on the electrical properties as a varistor. The phase identification and morphology of the ceramics were investigated by x–ray diffraction(XRD) and scanning electron microscope (SEM), respectively. A high voltage measuring unit and precision impedance analyzer were used to determine the nonohmic (J–E) behaviors and measure the dielectric properties and impedance spectroscopy of the ceramics, respectively. The results showed that RE oxides enhanced greatly the breakdown electric flied but reduced the nonlinear coefficient and the mean grain size of CCTO ceramics. There was a good linear relationship between ln J and E 1/2, which demonstrated that the Schottky barrier should exist at the grain boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. The depletion layer width determined by diffusion distance of RE ions and the effective surface states played important roles on the electrical properties of the ceramics.
84 citations
TL;DR: In this article, the authors investigate layer systems consisting of a thin layer of thermally grown SiO2 and different dielectric capping layers deposited by means of plasmaenhanced chemical vapor deposition (PECVD).
Abstract: For the passivation of p-type silicon surfaces, we investigate layer systems consisting of a thin layer of thermally grown SiO2 and different dielectric capping layers deposited by means of plasma-enhanced chemical vapor deposition (PECVD). We find that the thermal SiO2 layer thickness strongly impacts the passivation quality and interface parameters of the stacks. Capacitance-voltage measurements reveal that for Al2O3 and SiNx capping layers, an increased thermal SiO2 film thickness suppresses charge formation at the interface between SiO2 and the capping layer. Interface trap density and effective carrier lifetime data suggest that a certain thermal SiO2 thickness is required to achieve appropriate chemical passivation. The combination of a thin thermal SiO2 layer (~4 nm) and a PECVD-SiOx capping results in very low surface recombination velocities of a few centimeters per second, measured on p-type 1-Ω·cm float-zone silicon after contact firing and postmetallization annealing. The experimentally observed dependence of the surface recombination velocity on the fixed charge density, gate voltage, and injection density is reproduced very accurately by analytical calculations that use the measured interface trap density and total charge density at the Si/insulator interface. The model also includes additional recombination in the space charge region of inverted surfaces.
77 citations
TL;DR: A novel approach for the fabrication of transistors and circuits based on individual single-crystalline ZnO nanowires synthesized by a low-temperature hydrothermal method using a self-assembled monolayer dielectric leads to a reduction of the gate current by more than 3 orders of magnitude.
Abstract: A novel approach for the fabrication of transistors and circuits based on individual single-crystalline ZnO nanowires synthesized by a low-temperature hydrothermal method is reported. The gate dielectric of these transistors is a self-assembled monolayer that has a thickness of 2 nm and efficiently isolates the ZnO nanowire from the top-gate electrodes. Inverters fabricated on a single ZnO nanowire operate with frequencies up to 1 MHz. Compared with metal-semiconductor field-effect transistors, in which the isolation of the gate electrode from the carrier channel relies solely on the depletion layer in the semiconductor, the self-assembled monolayer dielectric leads to a reduction of the gate current by more than 3 orders of magnitude.
73 citations
TL;DR: In this article, N-polar III-nitride quantum-well ultraviolet light-emitting diodes are grown by plasma-assisted molecular beam epitaxy that integrate polarization-induced p-type doping by compositional grading from GaN to AlGaN along N-face.
Abstract: Nitrogen-polar III-nitride heterostructures present unexplored advantages over Ga(metal)-polar crystals for optoelectronic devices. This work reports N-polar III-nitride quantum-well ultraviolet light-emitting diodes grown by plasma-assisted molecular beam epitaxy that integrate polarization-induced p-type doping by compositional grading from GaN to AlGaN along N-face. The graded AlGaN layer simultaneously acts as an electron blocking layer while facilitating smooth injection of holes into the active region, while the built-in electric field in the barriers improves carrier injection into quantum wells. The enhanced doping, carrier injection, and light extraction indicate that N-polar structures have the potential to exceed the performance of metal-polar ultraviolet light-emitting diodes.
73 citations
TL;DR: Dynamic measurements of the electric field gradient are reported showing that it is stable over time and that its axial position in the microchannel is directly correlated to the location of an enriched tracer band.
Abstract: Bipolar electrode (BPE) focusing is a developing technique for enrichment and separation of charged analytes in a microfluidic channel. The technique employs a bipolar electrode that initiates faradaic processes that subsequently lead to formation of an ion depletion zone. The electric field gradient resulting from this depletion zone focuses ions on the basis of their individual electrophoretic mobilities. The nature of the gradient is of primary importance to the performance of the technique. Here, we report dynamic measurements of the electric field gradient showing that it is stable over time and that its axial position in the microchannel is directly correlated to the location of an enriched tracer band. The position of the gradient can be tuned with pressure-driven flow. We also show that a steeper electric field gradient decreases the breadth of the enriched tracer band and therefore enhances the enrichment process. The slope of the gradient can be tuned by altering the buffer concentration: higher concentrations result in a steeper gradient. Coating the channel with the neutral block co-polymer Pluronic also results in enhanced enrichment.
TL;DR: In this article, a polycrystalline ZnGa 2 Se 4 thin film was prepared by thermal evaporation technique on n-Si wafer followed by annealing at 700 K. The results show that the Al/p-ZnGa2 Se 4 /n-Si/Al heterojunction is a good candidate for the electronic device applications.
TL;DR: In this article, a multi-layer P+PN+ junction was designed and fabricated for betavoltaic batteries, and the design was optimized according to the characteristics of GaAs interface states and the diffusion length in the depletion region of the GaAs carriers.
Abstract: GaAs junctions are designed and fabricated for betavoltaic batteries. The design is optimized according to the characteristics of GaAs interface states and the diffusion length in the depletion region of GaAs carriers. Under an illumination of 10 mCi cm−2 63Ni, the open circuit voltage of the optimized batteries is about ~0.3 V. It is found that the GaAs interface states induce depletion layers on P-type GaAs surfaces. The depletion layer along the P+PN+ junction edge isolates the perimeter surface from the bulk junction, which tends to significantly reduce the battery dark current and leads to a high open circuit voltage. The short circuit current density of the optimized junction is about 28 nA cm−2, which indicates a carrier diffusion length of less than 1 µm. The overall results show that multi-layer P+PN+ junctions are the preferred structures for GaAs betavoltaic battery design.
Patent•
11 Jul 2011TL;DR: In this article, an E-mode high electron mobility transistor (HEMT) with a 2Dimensional Electron Gas (2DEG) barrier and a gate electrode is described.
Abstract: An Enhancement-mode (E-mode) high electron mobility transistor (HEMT) includes a channel layer with a 2-Dimensional Electron Gas (2DEG), a barrier layer inducing the 2DEG in the channel layer, source and drain electrodes on the barrier layer, a depletion layer on the barrier layer between the source and drain electrodes, and a gate electrode on the depletion layer. The barrier layer is recessed below the gate electrode and the depletion layer covers a surface of the recess and extends onto the barrier layer around the recess.
TL;DR: In this article, the forward I-V characteristics of 4H-SiC p-i-n diodes are studied in a wide range of currents and temperatures by means of an analytical model that allows us to highlight the minority current contributions in various diode regions, namely, the highly doped regions, the neutral base, and the space charge layer.
Abstract: The forward I-V characteristics of 4H-SiC p-i-n diodes are studied in a wide range of currents and temperatures by means of an analytical model that allows us to highlight the minority current contributions in various diode regions, namely, the highly doped regions, the neutral base, and the space charge layer. By accounting for the doping dependence of various physical parameters, such as bandgap narrowing, incomplete doping activation, carrier lifetime, and mobility, the model turns useful to investigate the role of various material properties at different current levels and temperatures. The accuracy of the model is verified by comparisons with numerical simulations and experimental data in a wide range of currents and temperatures, so that this model turns very useful for better understanding the impact of technological parameters on the steady-state behavior of diodes and obtaining an accurate circuital model of diodes.
Patent•
09 Aug 2011TL;DR: In this article, a gate-dielectric is interfaced directly onto the drain-tunneling region such that the drain tunneling region is between the source tunneling regions and the gate dielectric, so that the gate is configured to impose an external electric field which is oriented in parallel to the internal electric field of the depletion region.
Abstract: A low voltage tunnel field effect transistor includes a p-n tunnel junction, a gate-dielectric, a gate, a source-contact, and a drain-contact. The p-n tunnel junction includes a depletion region interfacing together a source-layer and a drain-layer. The depletion region includes a source-tunneling-region of the source-layer and a drain-tunneling-region of the drain-layer. When no external electric field is imposed, the depletion region of the p-n tunnel junction has an internal electric field that substantially points towards the source-tunneling-region and the drain-tunneling-region. The gate-dielectric is interfaced directly onto the drain-tunneling-region such that the drain-tunneling-region is between the source-tunneling-region and the gate-dielectric. The gate is interfaced onto the gate-dielectric such that the gate is configured to impose an external electric field which is oriented substantially in parallel to the internal electric field of the depletion region.
TL;DR: Carrier depletion and transport in a single ZnO nanowire Schottky device have been investigated at 5 K, using cathodoluminescence measurements, found to increase linearly with the reverse bias.
Abstract: Carrier depletion and transport in a single ZnO nanowire Schottky device have been investigated at 5 K, using cathodoluminescence measurements. An exciton diffusion length of 200 nm has been determined along the nanowire axis. The depletion width is found to increase linearly with the reverse bias. The origin of this unusual dependence in semiconductor material is discussed in terms of charge location and dimensional effects on the screening of the junction electric field.
TL;DR: In this article, a multiple-recessed 4H-SiC MESFET with modified depletion region is introduced, where the lower gate is divided into a number (N) of smaller step-shaped sections.
Abstract: In this paper, a novel 4H–SiC metal semiconductor field effect transistor (MESFET) with modified depletion region is introduced. The key idea in this work is modifying the depletion region in the channel for improving the electrical performances. The proposed structure consists of upper and lower gates. Also, the lower gate is divided into a number (N) of smaller step-shaped sections. Therefore, we have called the proposed structure multiple-recessed 4H–SiC MESFET (MR-MESFET). DC and RF characteristics of the MR-MESFET structure with various lower gate segments are analyzed by 2D numerical simulation. The simulated results show that as the number of the lower gate sections increases, the channel depletion region is modified and the drain current ( I D ) enhances. Also, by increasing the number of the lower gate sections, the breakdown voltage ( V BR ) enhances, too. Improvement of the I D and V BR leads to a further increase in the output power density of the device. Also, cut-off frequency ( f T ), maximum oscillation frequency ( f max ), and maximum available gain (MAG) improvements are achieved for the MR-MESFET structure with further number of the lower gate sections. The results show that the MR-MESFET structure with higher number of the lower gate segments has superior electrical characteristics and performances in comparison with the MR-MESFET structure with fewer number of the lower gate sections.
TL;DR: In this paper, the electrochemical parameters of TiO2 nanoribbons and nanoparticle-based electrodes were analyzed by electrochemical impedance spectroscopy, and it was shown that the intrinsic one-dimensional crystalline structure of the polygonal structure of a polysilicon semiconductor can promote formation of a space charge layer on the surface of the semiconductor, which effectively blocks the recombination of electrons with I-3(-) ions in the electrolyte interface.
Abstract: In dye-sensitized solar cells, there is a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and back-reaction of electrons to recombination with I-3(-) ions on the semiconductor electrolyte interface, which determines the charge collection efficiency and is strongly influenced by the electronic site distribution in intraband and geometrical structure of the semiconductors. Herein, we systematically analyze the electrochemical parameters of TiO2 nanoribbon- and nanoparticle-based electrodes by electrochemical impedance spectroscopy. The results show that the intrinsic one-dimensional crystalline structure of TiO2 nanoribbons can promote formation of a space charge layer on the surface of the semiconductor, which effectively blocks the recombination of electrons with I-3(-) ions in the semiconductor electrolyte interface, resulting in an increase of electron lifetime and a higher cell voltage. Furthermore, the boundaryless structure of the TiO2 nanoribbons provides efficient channels for electron transport and therefore increases electron diffusion length. The combination of TiO2 nanoparticle-based electrode with TiO2 nanoribbons can significantly improve energy conversion efficiency of similar to 60%. These data provide a basic understanding of the role of TiO2 geometrical structure in solar energy conversion.
TL;DR: In this article, the authors investigated the temperature dependence of carrier transport and resistance switching of Schottky junctions in the temperature range 80--400 K by measuring the currentvoltage (I-V) characteristics and the frequency dependence of the capacitance-voltage characteristics.
Abstract: We investigated the temperature dependence of carrier transport and resistance switching of $\mathrm{Pt}/\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ Schottky junctions in the temperature range 80--400 K by measuring the current-voltage (I-V) characteristics and the frequency dependence of the capacitance-voltage (C-V) characteristics. The I-V curves displayed a high degree of hysteresis, known as the colossal electroresistance (CER) effect, and their temperature dependence showed an anomalous behavior, i.e., the magnitude of the hysteresis increased with decreasing $T$. The experimental results were analyzed by taking into account the temperature and electric-field dependence of the relative permittivity of $\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ as well as the inhomogeneity of the Schottky barrier height (SBH) (a model in which two parallel current paths coexist in the Schottky barrier). It was confirmed that the observed I-V and C-V curves were well simulated by this model, thus indicating that the CER effects originated in the field emission current through different SBHs and at different locations of the Schottky junctions. Based on these results, we explain the mechanism of the CER effect qualitatively in terms of this model. For this purpose, we take into account the pinched-off effect caused by the small-scale inhomogeneity of SBH and the existence of deep levels as a result of defects and unintentional impurities in the depletion layer of the $\mathrm{Pt}/\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ Schottky junctions.
TL;DR: In this paper, a detailed electro-analytical framework for direct determination of a broad range of performance-indicator parameters of silicon solar cells is presented, including capacitances, diode resistance, series resistance, concentration of majority carriers as well as lifetime of minority carriers in the base, resistance and capacitance of the back surface field junction, and relaxation time of holes in the BSF.
Abstract: This work reports a detailed electro-analytical framework for direct determination of a broad range of performance-indicator parameters of silicon solar cells. A mono-crystalline Si cell, equipped with the efficiency-boosting back surface field (BSF) feature, is used as a model system for this study. Independently controlled illumination (0–1 Sun, from a solar simulator) and temperature of the cell prevent the interference of photothermal and photovoltaic processes during the measurements. The technique of D.C. voltammetry is employed to obtain current–voltage plots, fill-factors, efficiencies and effective cell resistances. The same experimental platform also supports A.C. impedance spectroscopic probing of the solar cell, which, in combination with complex nonlinear least square analysis of the experimental data, provides detailed information about both the emitter–base and the BSF components of the photovoltaic device. These impedance measurements lead to straightforward determination of the diffusion and depletion layer capacitances, diode resistance, series resistance, concentration of majority carriers as well as lifetime of minority carriers in the base, resistance and capacitance of the BSF junction, and relaxation time of holes in the BSF. The results presented here demonstrate how relatively simple electro-analytical experiments can be strategically utilized for quantitative characterization of photovoltaic systems.
Patent•
07 Mar 2011TL;DR: In this article, the p-type junction barrier region includes a polysilicon region forming a P-N heterojunction with the drift region, and the P-type barrier region is electrically connected to the contact.
Abstract: An electronic device includes a silicon carbide layer including an n-type drift region therein, a contact forming a junction, such as a Schottky junction, with the drift region, and a p-type junction barrier region on the silicon carbide layer. The p-type junction barrier region includes a p-type polysilicon region forming a P-N heterojunction with the drift region, and the p-type junction barrier region is electrically connected to the contact. Related methods are also disclosed.
TL;DR: In this paper, the electrical properties of an organic Schottky diode based on fluorescein sodium salt were investigated by current density-voltage and capacitancevoltage measurements.
TL;DR: Measurements show that charge transfer associated with chemisorption of oxygen in different forms produces an upward band bending and leads to an increase in the depletion layer width by approximately 70 nm, thereby decreasing surface conductivity and forming the basis for the molecular sensing capability of the nanowires.
Abstract: We report a study of the role of mid-gap defect levels due to surface states in SnO(2) nanowires on carrier trapping. Ultrafast pump-probe spectroscopy provides carrier relaxation time constants that reveal the nature and positions of various defect levels due to the surface states which in turn provide details on how the carriers relax after their injection. The effect of oxygen annealing on carrier concentration is also studied through XPS valence band photoemission spectroscopy, a sensitive non-contact surface characterization technique. These measurements show that charge transfer associated with chemisorption of oxygen in different forms produces an upward band bending and leads to an increase in the depletion layer width by approximately 70 nm, thereby decreasing surface conductivity and forming the basis for the molecular sensing capability of the nanowires.
01 Dec 2011-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, the effect of carrier avalanche multiplication in the space charge region has been investigated in detectors irradiated to 1015 −1016 −1016 cm2 and it is shown that the efficient trapping of free carriers from the bulk generation current to the deep levels of radiation induced defects leads to the stabilization of the irradiated detector operation in avalanche multiplication mode.
Abstract: The model explaining an enhanced collected charge in detectors irradiated to 1015–1016 neq/cm2 is developed. This effect was first revealed in heavily irradiated n-on-p detectors operated at high bias voltage ranging from 900 to 1700 V. The model is based on the fundamental effect of carrier avalanche multiplication in the space charge region and in our case is extended with a consideration of p–n junctions with a high concentration of the deep levels. It is shown that the efficient trapping of free carriers from the bulk generation current to the deep levels of radiation induced defects leads to the stabilization of the irradiated detector operation in avalanche multiplication mode due to the reduction of the electric field at the junction. The charge collection efficiency and the detector reverse current dependences on the applied bias have been numerically simulated in this study and they well correlate to the recent experimental results of CERN RD50 collaboration. The developed model of enhanced collected charge predicts a controllable operation of heavily irradiated detectors that is promising for the detector application in the upcoming experiments in a high luminosity collider.
TL;DR: In this article, a special test structure (front side metallization test pattern) was developed and applied on Cz-Si H-patterned solar cells to ensure a quantitative evaluation of SCR-recombination.
TL;DR: Picosecond acoustic pulses generated by femtosecond laser excitation of a metal film induce a transient current with subnanosecond rise time in a GaAs/Au Schottky diode.
Abstract: Picosecond acoustic pulses generated by femtosecond laser excitation of a metal film induce a transient current with subnanosecond rise time in a $\mathrm{GaAs}/\mathrm{Au}$ Schottky diode. The signal consists of components due to the strain pulse crossing the edge of the depletion layer in the GaAs and also the $\mathrm{GaAs}/\mathrm{Au}$ interface. A theoretical model is presented for the former and is shown to be in very good agreement with the experiment.
Patent•
28 Dec 2011TL;DR: In this article, an optical modulator with a depletion region comprising a junction between from a first conductivity type portion and a second conductivities type portion, a first intrinsic region, and another intrinsic region is provided.
Abstract: According to embodiments of the present invention, an optical modulator is provided. The optical modulator includes a depletion region comprising a junction between from a first conductivity type portion and a second conductivity type portion, a first intrinsic region, and a second intrinsic region, and wherein the depletion region is disposed between the first intrinsic region and the second intrinsic region.
TL;DR: In this paper, the authors examined the space charge layer formation at metal/ionic conductor interfaces by solving Poisson's equation using defect energetics calculated from first principles, and they found that oxidizing atmosphere and high valence band offset resulted in negative space charge accumulation at the interface, while reducing atmosphere and low valence bands offset result in positive charge accumulation.
TL;DR: In this article, three p-i-n GaAs solar cells were grown and characterized, one with InAs quantum dot (QD) layers embedded in the depletion region (sample A), one with QD layers embedded on the n− base region (B), and the third without QDs (control sample C).
Abstract: In this paper, three p–i–n GaAs solar cells were grown and characterized, one with InAs quantum dot (QD) layers embedded in the depletion region (sample A), one with QD layers embedded in the n− base region (B), and the third without QDs (control sample C). QD-embedded solar cells (samples A and B) show broad photoluminescence spectra due to QD multi-level emissions but have lower open-circuit voltages Voc and lower photovoltaic (PV) efficiencies than sample C. On the other hand, the short-circuit current density Jsc in sample A is increased while it is decreased in sample B. Theoretical analysis shows that in sample B where the built-in electric field in QDs is zero, electrons tend to occupy QDs and strong potential variations exist around QDs which deteriorate the electron mobility in the n− base region so that Jsc in sample B is decreased. Hole trapping and electron–hole recombination in QDs are also enhanced in sample B, resulting in a reduced Voc and thus a worse PV effect. In sample A, a strong built-in field exists in QD layers, which facilitates photo-carrier extraction from QDs and thus Jsc is increased. However, QDs in the depletion region in sample A act also as recombination-generation centers so that the dark saturated current density is drastically increased, which reduces Voc and the total PV effect. In conclusion, a nonzero built-in electric field around QDs is vital for using QDs to increase the PV effect in conventional p–i–n GaAs solar cells.