Built-in Potential Difference Research Articles

Homoepitaxial Si-doped Gallium Oxide films by MOCVD with tunable electron concentrations and electrical properties

Doping technology with stable carrier concentration regulation is a guarantee for optimizing β-Ga2O3-based high-power electronic devices and optoelectronic devices. In this study, β-Ga2O3 homoepitaxial epitaxial films with stable and tunable electron concentrations were grown on Fe-doped (010)-oriented β-Ga2O3 substrates by using metal-organic chemical vapor deposition (MOCVD). XRD shows that the basic bulk properties of β-Ga2O3 film, i. e. the structure and crystal orientation, are stable with low Si-doping, however, its electrical properties vary remarkably depending on concentrations of Si ions incorporated by Si source flow rate during the film growth process. Hall measurement is utilized to confirm high-quality β-Ga2O3 films with tunable electron concentration of 1017–1019 cm−3. As a consequence, the electron concentration-dependent behavior of β-Ga2O3 Schottky barrier diodes (SBDs) was studied. The β-Ga2O3 SBDs exhibited better forward current output electrical performance with reduction in built-in potential difference (Vbi) from 2.4 V to 0.3 V and on-resistance (Ron) from 257 Ω cm to 26 Ω cm. Our results suggest that stable and tunable electron concentration provides high-quality material assurance for constructing power electronic devices of Ga2O3.

Novel Backside Structure for Reverse Conducting Insulated-Gate Bipolar Transistor With Two Different Collector Trench

A novel reverse-conducting insulated gate bipolar transistor (RC-IGBT) with two different collector trench (DCT) is proposed. One of the collector trenches is filled with heavily doped N-type polysilicon (N-poly) and the other is filled with heavily doped N- and P-poly. An electron accumulation layer is formed along the sidewall of trench owing to built-in potential difference between the N-poly and the N-drift region. The electron accumulation layer and collector trenches block the electric field, which behaves just like the N-buffer layer of the conventional RC-IGBT (Con. RC-IGBT). Similarly, due to potential difference between the P-poly and N-drift region, a high-density hole inversion layer is formed to narrow the electron current path as a high resistance. The DCT RC-IGBT achieves snapback-free in a small cell pitch without additional control. Moreover, the DCT RC-IGBT shows better tradeoff between turn-off loss and ON-state voltage drop than that of con. RC-IGBT. For the same forward voltage drop, the turn-off loss is reduced by 26%.

Self-Driven Infrared Electrochromic Device with Tunable Optical and Thermal Management

Electrochromic devices (ECDs) exhibiting tunable optical and thermal modulation in the infrared (IR) region have attracted extensive attention in recent years due to their attractive application prospects in both military and civilian settings. However, considering the continuous energy supply needed for driving the device operation, it is desired to develop advanced IR-ECDs with low energy consumption. Herein, a flexible self-driven IR-ECD is constructed for achieving variable optical and thermal management in a low-energy mode. In this device, a built-in potential difference of 1.36 V exists between the EC polyaniline cathode and the aluminum foil anode. Consequently, there is a rapid and obvious increase in the IR reflectance of the device after connecting the two electrodes. Such a self-driven reflectance contrast is over 20% at the wavelength of 1500 nm, and the coloration efficiency of the device reaches up to 93.6 cm2 C-1. Meanwhile, the maximum apparent temperature modulation on the surface of the device reaches up to 5.6 °C. Then, the self-driven IR-ECD could recover to its original state driven by a solar cell, indicating good reversibility and stability. We anticipate that this work may provide a new insight into developing advanced self-driven IR-ECDs for applications in dynamic military camouflage and commercial thermal control.

WSe2 Homojunction p-n Diode Formed by Photoinduced Activation of Mid-Gap Defect States in Boron Nitride.

A single nanoflake lateral p-n diode (in-plane) based on a two-dimensional material can facilitate electronic architecture miniaturization. Here, a novel lateral homojunction p-n diode of a single WSe2 nanoflake is fabricated by photoinduced doping via optical excitation of defect states in an h-BN nanoflake upon illumination. This lateral diode is fabricated using a mechanical exfoliation technique by stacking the WSe2 nanoflake partially on the h-BN and Si substrates. The carrier type in the part of the WSe2 film on the h-BN substrate is inverted and a built-in potential difference is formed, ranging from 5.0 to 4.50 eV, which is measured by Kelvin probe force microscopy. The contact potential difference across the junction of p-WSe2 and n-WSe2 is found to be ∼492 mV. The lateral diode shows an excellent rectification ratio, up to ∼3.9 × 104, with an ideality factor of ∼1.1. A typical self-biased photovoltaic behavior is observed at the p-n junction upon the illumination of incident light, that is, a positive open-circuit voltage (Voc) is generated, that is, voltage obtained (at Ids = 0 V), and also a negative short-circuit current (Isc) is generated, that is, current obtained (at Vds = 0 V). The presence of built-in potential in the proposed homojunction diode establishes Isc and Voc upon illumination, which can be implemented for a self-powered photovoltaic system in future electronics. The proposed doping technique can be effectively applied to form planar homojunction devices without a photoresist for future electronic and optoelectronic applications.

Improved short-circuit current density in bulk heterojunction solar cells with reduced graphene oxide-germanium dioxide nanocomposite in the photoactive layer

In the quest to improve the optical absorption and electrical transport of poly-3-hexylthiophene (P3HT) and (6-6) phenyl-C61-butyric acid methyl ester (PCBM) blend film, reduced graphene oxide-germanium dioxide nanocomposite (rGO-GeO2) was employed in the photoactive layer of thin film organic solar cells. Bulk heterojunction solar cells (BHJ SCs) with rGO-GeO2 composite in the active layer exhibited an increase in power conversion efficiency (PCE) of up to 53%. Significant improvement in the measured photocurrent is achieved by the incorporation of rGO-GeO2 in the active layer. High short-circuit current density (Jsc) of up to 17 mA/cm2 is attained in the BHJ SCs. The high Jsc shows that the inlay of rGO-GeO2 in the active layer facilitates exciton separation and creates percolation pathways for charge transport to the electrodes. Charge separation is energetically favoured by a built-in potential difference between the donor and acceptor phases of the active layer. Hence, the incorporation of rGO-GeO2 composite in the active layer improves its charge photogeneration, separation and transport to yield high Jsc and enhanced PCE.

Temperature-dependent time response characteristic of photovoltaic performance in planar heterojunction perovskite solar cell

In recent years, perovskite solar cell (PSC) has achieved power conversion efficiency as high as over 20 %, making it competitive with high-efficiency thin film solar cells such as CuInGaSe and CdTe solar cells. However, the critical issue of reliability and stability for PSC should be addressed since a significant degradation of photovoltaic (PV) performance at low temperature has been found regardless of planar mesoporous PSC. To reveal the degradation of PV performance in PSC, the temperature-dependent PV performance of the planar PSC is investigated in detail. A PSC sample is loaded into a cryostat chamber connected to a compressor and illuminated by a halogen lamp. The operating temperature varies from 200 K to 325 K and the current-voltage (J-V) characteristic of planar PSC is measured at different scan rates from 10 V/s to 0.0017 V/s. At a fast scan rate of 10 V/s, the PSC shows a low PV performance at either low temperature or high temperature. The short-circuit current (JSC), open-circuit voltage (VOC) and maximum power point (PMPP) are found to decline with the temperature decrteasing. Moreover, the J-V curve also shows the S-shape characteristic. This suggests that the inefficient transport of photo-generated carriers occurs in the PSC. Ions such as Pb2+, CH3NH3+ and I-vacancies cause the screening effect of built-in field and the photo-generated carriers cannot be separated nor collected efficiently. As a result, JSC and VOC show small values in J-V curves measured at a fast scan rate. However, the degradation in PV performance is temporary. The PV performance gradually reaches a steady state at different operating temperatures with scan rate going down to 0.0017 V/s. The PMPP and VOC increase with temperature decreasing. These results indicate that a long illumination time is necessary for PSC to reach a steady state. After long-time illumination under biased condition (i.e., J-V curves measured at slow scan rate), the built-in field is compensated for by the external bias and the ions piling in the interface regions have enough time to diffuse towards the opposite direction. Thus, the screening effect of built-in field is reduced and the PV performance of PSC reaches a steady state. According to the result of device simulation, the increasing VOC at low temperature is attributed to the enhanced built-in potential difference and the reduced recombination rate of carriers. The temperature-dependent external quantum efficiency measurements of planar PSC before and after light illuminationis are performed to investigate the mechanism of carrier transport. It reveals that the separation and collection efficiencies of photo-generated carriers can be improved significantly after light illumination due to the fact that the screening effect of built-in field is reduced. These findings help understand the carrier transport mechanism in planar PSC.

Bipolar doping of double-layer graphene vertical heterostructures with hydrogenated boron nitride.

Using first-principles calculations, we examined the bipolar doping of double-layer graphene vertical heterostructures, which are constructed by hydrogenated boron nitride (BN) sheets sandwiched into two parallel graphene monolayers. The built-in potential difference in hydrogenated BN breaks the interlayer symmetry, resulting in the p- and n-type doping of two graphene layers at 0.83 and -0.8 eV, respectively. By tuning the interlayer spacing between the graphene and hydrogenated BN, the interfacial dipole and screening charge distribution can be significantly affected, which produces large modulations in band alignments, doping levels and tunnel barriers. Furthermore, we present an analytical model to predicate the doping level as a function of the average interlayer spacing. With large interlayer spacings, the "pillow effect" (Pauli repulsion at the highly charge overlapped interface) is diminished and the calculated Dirac point shifts are in good accordance with our prediction models. Our investigations suggest that this double-layer graphene heterostructures constructed using two-dimensional Janus anisotropic materials offer exciting opportunities for developing novel nanoscale optoelectronic and electronic devices.

Band-gap modulation in hydrogenated graphene/boron nitride heterostructures: The role of heterogeneous interface

First-principles calculations based on all-electron density functional theory are performed on hydrogenated graphene/boron nitride (BN) bilayer and graphene/BN multilayers to exploit the effects of the heterogeneous interface and external bias voltage on controlling their electronic properties. After hydrogenation, the emerging of the dipoles at the interface induced by the charge transfer between the graphene and the BN layer introduces a built-in potential difference, which plays a critical role in determining the band gaps of the resulting system. Tuning this internal potential difference through changing the number of BN layers or an external bias allows continuous modulation of the band gaps and leads to a linear transition from semiconductors to metals.

1A1-V01 環境発電素子を目指したアルカリエレクトレット膜の高電圧化に関する検討(MEMSとナノテクノロジー)

Silicon dioxide electret induced by highly doped potassium ions will be demonstrated by forming on a comb-drive actuator. The comb-drive actuator made of silicon on insulator (SOI) substrate is oxidized with bubbling stream of KOH solution to form a silicon oxide film including potassium ions on the etched side-walls of comb electrodes uniformly. After bias-temperature (BT) procedure at about 900-1000 K and 100 V was applied to the device, we have confirmed 40 V built-in potential difference between the opposing comb electrodes. In order to obtain higher potential electret film, we have fabricated a "glass" comb-drive actuator.

Secondary electron dopant contrast imaging of compound semiconductor junctions

Secondary electron imaging combined with immersion lens and through-the-lens detection has been used to analyze various semiconductor junctions. Dopant contrast imaging was applied for multi-doped 4H–SiC, growth-interrupted n+/p and n/n+ homoepitaxial interfaces, and an AlGaAs/GaAs p-n junction light-emitting diode structure. Dopant contrast was explained by the local variation in secondary electron escape energies resulting from the built-in potential difference. The effect of varying electron affinity on contrast for the heterostructures is also discussed. The contrast profile of the n-doped AlGaAs compared reasonably well to the simulated valence bandedge energy using a previously determined efficiency of dopant ionization.

Realising wide bandgap P-SiC-emitter lateral heterojunction bipolar transistors with low collector–emitter offset voltage and high current gain: a novel proposal using numerical simulation

The authors report a novel method to reduce the collector–emitter offset voltage of the wide bandgap SiC-P-emitter lateral HBTs using a dual-bandgap emitter. In their approach, the collector–emitter offset voltage VCE(offset) is reduced drastically by eliminating the built-in potential difference between the emitter–base (EB) and collector–base (CB) junctions by using a SiC-on-Si P-emitter. It is demonstrated that the proposed dual-bandgap P-emitter HBT, together with the SiGe base and Schottky collector, not only has a very low VCE(offset) but also exhibits high current gain, reduced Kirk effect, excellent transient response and high cutoff frequency. The performance of the proposed device is evaluated in detail using two-dimensional device simulation, and a possible BiCMOS compatible fabrication procedure is also suggested.

Local characterization of compositionally graded Pb(Zr,Ti)O 3 thin films by scanning force microscope

Compositionally graded PZT thin film was prepared by a sol–gel technique. Nanoscale domain structure and polarization reversal behavior in an individual grain were investigated by means of scanning force microscopy (SFM) in piezoresponse mode. Domain as small as 30 nm in size was visualized. The observed complex domain contrast was attributed to the crystallographic orientation of the grains and to the built-in potential difference in compositionally graded PZT thin film under an ac field. The built-in potential difference, defect and space charge gave rise to strong domain pinning behavior during the polarization reversal.

Photoelectric fields in semiconductors under strong excitation

Photoelectric fields induced in semiconductors with nonuniform impurity profiles cancel the built-in electric field. In the case of strong photoexcitation, the maximum open-circuit voltage approaches the internal built-in potential difference due to the impurity gradient between the ohmic contact probes on the semiconductor slice.