Hole Emission Research Articles

Graphene-insulator-semiconductor capacitors as superior test structures for photoelectric determination of semiconductor devices band diagrams

We report on the advantages of using Graphene-Insulator-Semiconductor (GIS) instead of Metal-Insulator-Semiconductor (MIS) structures in reliable and precise photoelectric determination of the band alignment at the semiconductor-insulator interface and of the insulator band gap determination. Due to the high transparency to light of the graphene gate in GIS structures large photocurrents due to emission of both electrons and holes from the substrate and negligible photocurrents due to emission of carriers from the gate can be obtained, which allows reliable determination of barrier heights for both electrons, Ee and holes, Eh from the semiconductor substrate. Knowing the values of both Ee and Eh allows direct determination of the insulator band gap EG(I). Photoelectric measurements were made of a series of Graphene-SiO2-Si structures and an example is shown of the results obtained in sequential measurements of the same structure giving the following barrier height values: Ee = 4.34 ± 0.01 eV and Eh = 4.70 ± 0.03 eV. Based on this result and results obtained for other structures in the series we conservatively estimate the maximum uncertainty of both barrier heights estimations at ± 0.05 eV. This sets the SiO2 band gap estimation at EG(I) = 7.92 ± 0.1 eV. It is shown that widely different SiO2 band gap values were found by research groups using various determination methods. We hypothesize that these differences are due to different sensitivities of measurement methods used to the existence of the SiO2 valence band tail.

Ballistic Hole Emission Spectroscopy of Self-Assembled GeSi/Si(001) Nanoislands

Ballistic hole emission microscopy/spectroscopy has been applied to study the electronic structure of the hole states in the self-assembled GeSi/Si(001) nanoislands. The ballistic hole emission microscopic images demonstrated the spots of increased collector current related to the ballistic electron tunnelling via the confined valence band states in the GeSi/Si(001) nanoislands. In the ballistic hole emission spectra of the Ge hut clusters the stepwise features attributed to the quantum confined hole states have been observed. The results of present study demonstrate the capabilities of the ballistic hole emission microscopy/spectroscopy in the characterization of the electronic structures of the valence band states in the GeSi/Si nanostructures.

InAs/InAsSb type-II strained-layer superlattices for mid-infrared LEDs

InAs/InAsSb type-II strained-layer superlattice (SLS) and multiple quantum well (MQW) structures have been studied for their suitability in the active region of mid-infrared LEDs operating at room temperature. A series of InAs/InAs1−xSbx superlattices with low antimony content (x = 3.8–13.5%) were grown by MBE on InAs substrates and characterised using x-ray diffraction and photoluminescence (PL). The 4 K PL spectra of these samples exhibit the expected peak shift to longer wavelength and a reduction in intensity as the Sb content is increased. Band structure simulations highlight the effects of changing the antimony content and the layer thicknesses, to tailor the overlap of the electron and hole wavefunctions and maximise the radiative recombination rate. Analysis of the temperature dependence of the PL emission spectra enabled the extraction of quenching energies that demonstrate some suppression of Auger recombination in both the MQW and SLS structures. The MQW samples exhibit a changeover in the dominant radiative recombination process above ~100 K associated with thermal emission of holes into the InAs barriers; this behaviour was not observed in the SLS samples. These SLS structures have the potential for use as the active region in room temperature mid-infrared LEDs.

Type-II GaSb/GaAs quantum-dot intermediate band with extended optical absorption range for efficient solar cells

GaSb/GaAs type-II quantum-dot solar cells (QD SCs) have attracted attention as highly efficient intermediate band SCs due to their infrared absorption. Type-II QDs exhibited a staggered confinement potential, where only holes are strongly confined within the dots. Long wavelength light absorption of the QDSCs is enhanced through the improved carriers number in the IB. The absorption of dots depends on their shape, material quality, and composition. Therefore, the optical properties of the GaSbGaAs QDs before and after thermal treatment are studied. Our intraband studies have shown an extended absorption into the long wavelength region $$1.77 \, \upmu \text {m}$$. The annealed QDs have shown significantly more infrared response of $$7.2 \, \upmu \text {m}$$ compared to as-grown sample. The photon absorption and hole extraction depend strongly on the thermal annealing process. In this context, emission of holes from localized states in GaSb QDs has been studied using conductance–voltage (G–V ) characteristics.

Ballistic hole emission spectroscopy of self-assembled GeSi/Si(001) nanoislands

AbstractBallistic hole emission microscopy/spectroscopy has been applied to study the electronic structure of the hole states in the self-assembled GeSi/Si(001) nanoislands. The ballistic hole emission microscopic images demonstrated the spots of increased collector current related to the ballistic electron tunnelling via the confined valence band states in the GeSi/Si(001) nanoislands. In the ballistic hole emission spectra of the Ge hut clusters the stepwise features attributed to the quantum confined hole states have been observed. The results of present study demonstrate the capabilities of the ballistic hole emission microscopy/spectroscopy in the characterization of the electronic structures of the valence band states in the GeSi/Si nanostructures.

Characterization of nitride hole lateral transport in a charge trap flash memory by using a random telegraph signal method

We use a random telegraph signal method to investigate nitride trapped hole lateral transport in a charge trap flash memory. The concept of this method is to utilize an interface oxide trap and its associated random telegraph signal as an internal probe to detect a local channel potential change resulting from nitride charge lateral movement. We apply different voltages to the drain of a memory cell and vary a bake temperature in retention to study the electric field and temperature dependence of hole lateral movement in a nitride. Thermal energy absorption by trapped holes in lateral transport is characterized. Mechanisms of hole lateral transport in retention are investigated. From the measured and modeled results, we find that thermally assisted trap-to-band tunneling is a major trapped hole emission mechanism in nitride hole lateral transport.

Fundamental constraints imposed by energy barriers on the fill factor and on the efficiency of silicon heterojunction solar cells

A model that relates the relatively low Fill Factor (FF) in p+/n amorphous/crystalline silicon (a-Si:H/c-Si) hetero-junction solar cells (SHJ) with perfectly ohmic contacts, to hole reflection at the hetero-interface under illumination is proposed. Such a reflection results from the partial emission and transport of holes imposed by the thermionic emission boundary condition at the hetero-interface. The non-emitted holes scatter back and diffuse towards the substrate. The probability of hole reflection depends on the thermionic emission parameters and also on the diffusion barrier associated with the pn junction adjacent to the hetero-interface. With such hole reflection the cell forward current under illumination is enhanced well above the cell dark current leading to Fill Factor and efficiency degradation. Such effects are found to be particularly significant in cells with deficient active doping in a-Si:H. The model is supported by the results of AFORS-HET simulations at room temperature and at different temperatures. When the model predictions are taken into account, p+/n SHJ cells with minimum hole reflection and FF's exceeding 81% could be designed.

The di‐interstitial in silicon: Electronic properties and interactions with oxygen and carbon impurity atoms

New experimental and theoretical results on the silicon di‐interstitial (I2) and its interactions with oxygen and carbon impurity atoms in Si crystals are reported. Electronic structure calculations indicate that I2 has an acceptor and a donor level in the gap, which are close to the conduction and the valence band edges, respectively. Experimental results, which support the theoretically predicted high mobility of I2, are discussed. It is argued that mobile I2 can be trapped by oxygen and carbon impurities. The I2O center has a donor level at Ev +0.09 eV. Two absorption bands at 936 and 929 cm−1 are assigned to the local vibrational modes of the I2O defect in the neutral and positively charged states, respectively. The binding energy of I2O relative to the separated I2 and Oi species is 0.22 eV. The disappearance of the I2O complex upon thermal annealing occurs in the temperature range 50–100 °C and is accompanied by the introduction of another defect, which gives rise to two hole emission signals from energy levels at Ev +0.54 and Ev +0.45 eV. It is argued that these levels are related to a complex consisting of interstitial carbon and interstitial silicon atoms (CiI). The stable configurations of the CiI pair have been found.

Non-Contact Photo-Assisted Charge-Based Characterization of Dielectric Interfaces in SiC: Evidence of Slow States

In this work we present novel photo-assisted characterization of dielectric interfaces in SiC using a modified non-contact corona-Kelvin technique. This technique eliminates the cost and time associated with fabrication of electrical test structures. UV illumination in deep depletion is used to generate minority carriers that empty deep interface states too slow to be emptied by thermal emission. After illumination, the interface state charging current is measured with time resolved voltage decay. This enables novel non-contact corona-Kelvin characterization of hole emission and electron capture processes involving slow interface states. This novel application complements standard corona-Kelvin measurement of dielectric, interface and semiconductor parameters.

Characterization of charge accumulation on multiple interfaces in phosphorescent organic light-emitting diodes

A series of green phosphorescent organic light emitting diodes (OLED) were studied using I(V), admittance spectroscopy, and capacitance-versus-voltage, C(V), measurements. We found that both the logarithmic derivative of I(V) and C(V) spectra revealed two distinct peaks related to the build-up and consequent dissipation of charge on different interfaces of the device. The first peak is common for many types of OLEDs and is caused by the external built-in potential. The second peak is a feature attributed to the specific stacking sequence of the devices studied. We argue that the secondary charge build-up occurs at the interface between hole transport layer and emission layer due to (i) the strong mismatch of HOMO level of these layers and (ii) restricted direct injection of holes onto the phosphorescent dopant molecules. Consequent dissipation of the charge is caused by greatly-enhanced supply of electrons, which in turn is caused by exponential growth of electron mobility due to the Frankel-Poole effect. Our study shows that C(V), in conjunction with I(V), measurements are not only useful for model devices with two metal-organic interfaces, but can also characterize charge accumulation in complex, multi-interface OLEDs. We also observed a strong negative contribution to the capacitance at low frequency and high biases; the emergence of the negative capacitance correlates with the onset of light emission.

Harvesting Triplet Excitons with Exciplex Thermally Activated Delayed Fluorescence Emitters toward High Performance Heterostructured Organic Light-Emitting Field Effect Transistors.

The utilization of triplet excitons plays a key role in obtaining high emission efficiency for organic electroluminescent devices. However, to date, only phosphorescent materials have been implemented to harvest the triplet excitons in the organic light-emitting field effect transistors (OLEFETs). In this work, we report the first incorporation of exciplex thermally activated delayed fluorescence (TADF) emitters in heterostructured OLEFETs to harvest the triplet excitons. By developing a new kind of exciplex TADF emitter constituted by m-MTDATA (4,4',4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine) as the donor and OXD-7 (1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene) as the acceptor, an exciton utilization efficiency of 74.3% for the devices was achieved. It is found that the injection barrier between hole transport layer and emission layer as well as the ratio between donor and acceptor would influence the external quantum efficiency (EQE) significantly. Devices with a maximum EQE of 3.76% which is far exceeding the reported results for devices with conventional fluorescent emitters were successfully demonstrated. Moreover, the EQE at high brightness even outperformed the result for organic light-emitting diode based on the same emitter. Our results demonstrate that the exciplex TADF emitters can be promising candidates to develop OLEFETs with high performance.

Physical characteristics of the radio emission of solar coronal holes

This paper presents a brief overview of works dedicated to the studies of solar coronal holes (CH). Special attention is paid to CH studies at millimeter and centimeter wavelengths. The observational data in the millimeter range, combined with satellite observations in the ultraviolet and soft X-ray range, as well as solar eclipse observations of March 29, 2006, at centimeter wavelengths with RATAN-600, provide new results for understanding the physical nature of coronal holes on the Sun.

Hole localization energy of 1.18 eV in GaSb quantum dots embedded in GaP

AbstractSelf‐organized GaSb quantum dots are embedded in GaP by molecular beam epitaxy and ‐diodes are fabricated. The structure of the sample is investigated using transmission electron microscopy with atomic resolution. The presence of quantum dots on top of a wetting layer and interdiffusion processes between Sb and P are observed. The localization energy, capture cross‐section, and storage time of holes in the ground state of the quantum dots are determined via deep‐level transient spectroscopy. Their localization energy of eV is found to agree with the theoretical prediction of 1.4 eV once the observed interdiffusion is taken into account. A storage time of holes of days at room temperature is calculated, marking an improvement of 3 orders of magnitude from previous record figures. GaSb/GaP quantum dots are, thus, promising candidates for future non‐volatile DRAMs or fast Flash.Schematic representation of the quantum dots (left) and of the valence band in their vicinity (right). The localization energy is marked on the scheme. The localization of the quantum dots prevents thermal emission of holes, generating a storage time at room temperature of 3.9 days.

Electrical Characterization of Electron Beam Induced Damage on Sub-10nm SRAM Using Nano-Probing Technique

Nano-probing techniques utilizing scanning electron microscopy (SEM) guidance have been widely employed in evaluation of electrical characteristics of the localized devices in the failure analysis of actual LSI systems [1] because of their high resolution imaging, probing accuracy and high throughput. As CMOS scaling is moving toward the sub-10 nm regime, electron beam induced damage is becoming a more critical issue in performing electrical characterization at the contact level using nano-probe techniques. However, the effects of electron beam induced damage have not been reported for sub-10 nm devices in the range of less than 1.0 kV of electron beam acceleration voltage (Vacc). In this paper, 10 nm design ruled n-channel metal-oxide-semiconductor transistors were prepared in order to investigate how much the transistors were affected by electron beam irradiation as a function of Vacc. Figure 1 illustrates ID - VG transfer characteristics of the transistors before and after electron beam irradiation with a 10 minutes exposure time at low (Vacc = 0.5 kV) and high (Vacc = 1.0 kV) Vacc. After electron beam irradiation, all device parameters shifted including threshold voltage, saturation current, sub-threshold slope and transistor leakage current. We observed different degradation characteristics in threshold voltage depending on Vacc. Figure 2 shows the percentage of threshold voltage shift (ΔVth) of the transistors before and after electron beam irradiation depending on Vacc with a 10 minutes exposure time. We also separated the threshold voltage shift (ΔVth) into the effect of oxide trapped charges (ΔVot) and interface traps (ΔVit), where ΔVth = ΔVot + ΔVit, using a technique based on sub-threshold current measurements [2] as shown in Fig. 2. A negative shift in threshold voltage occurred at low Vacc because of the increase of oxide trapped holes (ΔVot) generated by excited plasmons [3]. At high Vacc, however, a positive Vth shift was observed because of an increased contribution of interface trap generation (ΔVit) caused by the deeper electron penetration depth. In addition, interface trap generation degraded the sub-threshold slope because of the lower carrier mobility caused by generated interface traps. Another serious change observed in the electrical characteristics of the transistors after electron beam irradiation was an increase in transistor leakage current. Figure 3 illustrates the comparison between the change in transistor leakage current (ΔITr.off) and drain junction leakage current (ΔIDrain.off) before and after electron beam irradiation at various conditions. From the results, we conclude that ΔITr.off was caused by ΔIDrain.off and was related to the junction characteristic changes. As mentioned above, the electron beam produces interface traps by hole hopping and direct generation of incident electrons. These interface traps behave like generation-recombination (G-R) centers and can enhance carrier G-R rate in the space charge region (SCR) by four processes (electron capture, electron emission, hole capture and hole emission) [4, 5]. Finally, our studies showed that it is critical to avoid electron beam exposure before electrical device characterizations are carried out at the contact level using nano-probe systems even in the range of less than 1.0kV of Vacc because electron beam induced device parameter changes cannot be neglected during failure analysis of sub-10nm devices.

Experimental and Numerical Analysis of Hole Emission Process From Carbon-Related Traps in GaN Buffer Layers

The role of carbon-related traps in GaN-based ungated high-electron mobility transistor structures has been investigated both experimentally and by means of numerical simulations. A clear quantitative correlation between the experimental data and numerical simulations has been obtained. The observed current decrease in the tested structure during backgating measurements has been explained simply by means of a thermally activated hole-emission process with $E_{A} = 0.9$ eV, corresponding to the distance of the acceptor-like hole-trap level from the GaN valence band. Moreover, it has been demonstrated by means of electrical measurements and numerical simulations that only a low percentage of the nominal carbon doping levels induces the observed current reduction when negative substrate bias is applied to the tested structure.

Deep level study of Mg-doped GaN using deep level transient spectroscopy and minority carrier transient spectroscopy

Deep levels in Mg-doped GaN grown by metal organic chemical vapor deposition (MOCVD), undoped GaN grown by MOCVD, and halide vapor phase epitaxy (HVPE)-grown GaN have been studied using deep level transient spectroscopy and minority charge carrier transient spectroscopy on Schottky diodes. One hole trap, labeled HT1, was detected in the Mg-doped sample. It is observed that the hole emission rate of the trap is enhanced by increasing electric field. By fitting four different theoretical models for field-assisted carrier emission processes, the three-dimensional Coulombic Poole-Frenkel (PF) effect, three-dimensional square well PF effect, phonon-assisted tunneling, and one-dimensional Coulombic PF effect including phonon-assisted tunneling, it is found that the one-dimensional Coulombic PF model, including phonon-assisted tunneling, is consistent with the experimental data. Since the trap exhibits the PF effect, we suggest it is acceptorlike. From the theoretical model, the zero field ionization energy of the trap and an estimate of the hole capture cross section have been determined. Depending on whether the charge state is \ensuremath{-}1 or \ensuremath{-}2 after hole emission, the zero field activation energy ${E}_{i0}$ is 0.57 eV or 0.60 eV, respectively, and the hole capture cross section ${\ensuremath{\sigma}}_{p}$ is $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}$ or $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}$, respectively. Since the level was not observed in undoped GaN, it is suggested that the trap is associated with an Mg related defect.

Abrupt and tunable quenching of photoluminescence in ZnO

The abrupt and tunable quenching of photoluminescence (PL) in bulk ZnO samples is reported, a phenomenon which has been previously observed only in GaN and a few phosphorescent materials. ZnO samples grown by the hydrothermal method contain Li defects responsible for the orange luminescence (OL) band with a maximum at 2.0eV. The LiZn acceptor is well studied, but there is some disagreement in the literature on the value of its ionization energy. Here we show that the ionization energy of the LiZn acceptor responsible for the OL band is 0.65±0.10eV. The thermal emission of holes from the LiZn acceptor to the valence band and their subsequent capture by unknown nonradiative centers causes the abrupt quenching of PL, the temperature of which is tunable by excitation intensity.

Phonon‐assisted tunnel emission of holes from the double donor level of the EL2 defect

Strong electric‐field‐induced enhancement of the thermal emission rate of holes from the doubly ionised charge state of the ΕL2 defect was revealed with the double‐correlation deep‐level transient spectroscopy in p‐type GaAs crystals of various crystallographic orientations. The observed enhancement, by over four orders of magnitude of the emission rate, was analysed in a model of phonon‐assisted tunnel ionisation. Similar dependences of the hole‐emission rate on the electric‐field intensity received for the electric‐field directions parallel to three main crystallographic axes evidence for tetrahedral symmetry of the defect, which is consistent with its identification as the isolated arsenic antisite defect.

Improved Performance of Organic Light-Emitting Field-Effect Transistors by Interfacial Modification of Hole-Transport Layer/Emission Layer: Incorporating Organic Heterojunctions.

Organic heterojunctions (OHJs) consisting of a strong electron acceptor 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) and an electron donor N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) were demonstrated for the first time that they can be implemented as effective modification layers between hole transport layer (HTL) and emission layer in the heterostructured organic light-emitting field effect transistors (OLEFETs). The influence of both HAT-CN/NPB junction (npJ) and NPB/HAT-CN junction (pnJ) on the optoelectronic performance of OLEFETs were conscientiously investigated. It is found that both the transport ability of holes and the injection ability of holes into emissive layer can be dramatically improved via the charge transfer of the OHJs and that between HAT-CN and the HTL. Consequently, OLEFETs with pnJ present optimal performance of an external quantum efficiency (EQE) of 3.3% at brightness of 2630 cdm(-2) and the ones with npJs show an EQE of 4.7% at brightness of 4620 cdm(-2). By further utilizing npn OHJs of HAT-CN/NPB/HAT-CN, superior optoelectronic performance with an EQE of 4.7% at brightness of 8350 cdm(-2) and on/off ratio of 1 × 10(5) is obtained. The results demonstrate the great practicality of implementing OHJs as effective modification layers in heterostructured OLEFETs.

38-4: The Interfacial Effect between HTL and EML on the Efficiency of Solution Processed Green Phosphorescent OLEDs

We have fabricated highly efficient green phosphorescent organic light-emitting diodes by solution process. Very interestingly, we found that the device efficiency is affected by interface between hole transport layer (HTL) and emission layer (EML). It was proved through spin cleaning process of cross-linkable HTL after curing which may affect the interface mixing. In addition, we tried to add complement layer to manipulate such effects. As a result, we significantly improved device efficiency (up to 72.7 cd/A) after spin cleaning process of the cross-linkable HTL material.