Free Hole Density Research Articles

Study of the transient EL slow rise in single layer OLEDs

Abstract Study of the transient electroluminescence (EL) behavior at turn-on upon the application of a single voltage pulse is presented. Based on the continuity equations which describe the carrier dynamics, and on the recombination current which describe the EL output, we present a theoretical model to explain the slow rise of the EL associated to the electron packet motion in the bulk. The model enables us to determine, more than the mobility of majority carriers, the equilibrium density of free holes in the bulk. The dependence of free hole density ( p e ) of the applied voltage shows that the unipolar current flow for holes in the bulk follows as expected, an SCLC regime characterized by a linear dependence of p e with applied voltage. Our analysis allows us also to estimate the trap density (for holes) in the bulk of about 2 × 10 15 cm −3 . Both experimental and theoretical results are compared to each other and to other works, and were found to be consistent with the proposed model. The device under investigation is the conventional ITO/MEH-PPV/Al OLED.

Electrical transport properties of aluminum-implanted 4H–SiC

The free hole density and low-field mobility of aluminum-doped 4H–SiC were investigated in the temperature range of 100–900K, both, experimentally and theoretically. Experimental data for implanted p-type 4H–SiC were compared with theoretical calculations using parameters determined for high-quality epitaxial layers. The deformation potential for intra- and intervalley scattering by acoustic phonons and the effective coupling constant for intra- and intervalley scattering by nonpolar optical phonons were determined. The detailed analysis of the implanted layers with aluminum-targeted concentration ranging from 3.33×1018to1021cm−3 shows that (i) about half of the implanted atoms are electrically active in the SiC lattice, (ii) a systematic compensation of about 10% of the doping level is induced by the implantation process, (iii) two different ionization energies for the aluminum atoms have to be used. Their origin is discussed in terms of inequivalent hexagonal and cubic lattice sites. Finally, the doping dependence of the ionization ratio and Hall mobility are given for non- and weakly (10%) compensated material at 292K. The maximum achievable mobility for low-doped material in p-type 4H–SiC is shown to be 93cm2∕Vs at room temperature.

Some Aspects of Free Volume Studies in Molecular Substances Using Positron Annihilation Experiments

The first problem in the application of positron annihilation lifetime (PAL) spectroscopy for the study of elementary free volumes (EFV) in solids comes from inhibition of positronium formation, i.e. essential influence of small admixtures on the long-lived component intensity of the PAL spectrum. The nature of this effect and correlation of the phenomena with thermo-stimulated luminescence (TSL) are considered here along with calculation of the number density of free volume holes using characteristics of the PAL spectra in polyimides and systems with a highly developed specific surface, such as cross-linked polystyrenes (polymer sorbents) and silica based glasses. Finally, a special attention is paid to comparison of this value calculated on the bases of positron annihilation data with that found from sorption isotherms and Brunauer–Emmett–Teller (BET) theory.

A New Derivation of the Law of the Junctions

In this paper, the author presents students with a new and simple derivation of the law of the junctions. The law of the junctions is crucial to the correct understanding of the operation of the bipolar junction transistor. The integral of the product of the density of energy states and Fermi-Dirac distribution function is used to derive the density of free electrons and holes crossing from one side to another at a pn-junction under an external applied voltage.

Hydrogen control of ferromagnetism in a dilute magnetic semiconductor.

We show that upon exposure to a remote dc hydrogen plasma, the magnetic and electronic properties of the dilute magnetic semiconductor Ga1-xMnxAs change qualitatively. While the as-grown Ga1-xMnxAs thin films are ferromagnetic at temperatures T less, similar 70 K, the samples are found to be paramagnetic after the hydrogenation, with a Brillouin-type magnetization curve even at T=2 K. Comparing magnetization and electronic transport measurements, we conclude that the density of free holes p is significantly reduced by the plasma process, while the density of Mn magnetic moments does not change.

Prospects for Carrier‐Mediated Ferromagnetism in GaN

AbstractFor Abstract see ChemInform Abstract in Full Text.

Atomic structure of pyramidal defects in Mg-doped GaN

A detailed transmission electron microscopy study of pyramidal defects appearing in highly Mg-doped GaN is reported. It is shown that these defects are closed pyramidal inversion domains. From a high-resolution microscopy study, we propose atomic models for inversion domain boundaries which consist of ${\mathrm{Mg}}_{3}{\mathrm{N}}_{2}$ building blocks for both the basal and inclined facets of the pyramids. In Mg-doped GaN grown by metalorganics vapor phase epitaxy, these pyramidal inversion domains are a few nanometers wide, and their density is high enough to play a role in the free hole density decrease at high Mg doping.

Prospects for carrier‐mediated ferromagnetism in GaN

AbstractTheoretical predictions of room‐temperature ferromagnetism in Mn‐doped GaN and other wide band gap semiconductors suggest that these materials might be useful for spintronic applications. In this short review, we summarize recent observations on the gap states of GaN:Mn, which make it impossible that the two main prerequisites of these predictions can be fulfilled at the same time, which are (1) a large concentration of localized Mn2+ spins coexisting with (2) a high density of free holes in the valence band. Such conditions have been observed in only a few materials like e.g. GaAs:Mn. More typically, transition‐metal impurities act as traps for free carriers, thus pinning the Fermi level in the semiconductor band gap far from the valence or conduction band. Alternatively to ferromagnetism mediated by free carriers, the interactions between bound magnetic polarons and the double‐exchange mechanism have been suggested to possibly lead to ferromagnetism in GaN:Mn. Because of the energy position and the character of its gap states, Mn seems to be rather unsuitable for these two mechanisms. Better candidates would be GaN:Fe:Mg for a system of magnetic polarons, and GaN:Cr for a double‐exchange ferromagnet. Because of its short‐range nature, the double‐exchange interaction requires rather concentrated alloys and does not offer significant advantages of GaN:Mn over the established ferromagnetic materials. However, microscopic ferromagnetic inclusions observed in many SQUID measurements of GaN:Mn could possibly help to achieve spin injection in nitride semiconductors. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

Electrical investigation of carbon intrinsically‐doped GaAs layers grown by metalorganic vapour phase epitaxy from TMGa and TBAs

In this work the possibility of controlling carbon intrinsic-doping in GaAs homoepitaxial layers grown by metalorganic vapour phase epitaxy (MOVPE) from the trimethyl-gallium (TMGa) and tertiary-buthyl-arsine (TBAs) precursors was evaluated via the analysis of transport properties as a function of the growth parameters and for two substrates mis-orientations; Hall effect measurements were performed on the samples as a function of temperature. Intrinsically p-doped GaAs layers were obtained with a hole concentration in the range (1014–1018) cm–3 and a corresponding RT mobility in the range (100–400) cm2/Vs. The simultaneous analysis of the Hall free hole density and Hall mobility yielded the effective doping level, the compensation ratio and the thermal ionisation energy of the acceptor impurity as a function of the growth parameters.

Optical identification of impurity levels in strongly phosphorus‐doped wide‐gap II–VI bulk semimagnetic semiconductors

AbstractZn1–xMnxTe and Cd1–xMnxTe crystals doped with phosphorus have been investigated by means of the measurement of Hall effect, photoluminescence and reflectance. By annealing under high pressure of nitrogen (up to 4 MPa) at T = 800 °C for a week, a free hole density as high as 5 × 1018 cm–3 was achieved in Zn0.95Mn0.05Te, while in Cd0.99Mn0.01Te it was only 8 × 1016 cm–3. The phosphorus acceptor binding energy in Zn0.975Mn0.025Te and Cd0.99Mn0.01Te was found to be (0.036 ± 0.002) eV and (0.054 ± 0.002) eV, respectively.

Annealing-Induced Changes in Electrical, Optical, and Magnetic Properties of Phosphorus Doped Bulk Zn1-xMnxTe

We report on successful growth of heavily p-type doped bulk Zn 1-x Mn x Te crystals. Free hole density as high as 10 19 cm -3 has been achieved by use of Zn 3 P 2 as a source of phosphorus acceptors and by post-growth high pressure annealing. The latter proved to be of a vital importance, removing a strong compensation present in as-grown materials. As a result, the annealed crystals exhibit a very bright green photoluminescence (λ 520 nm) at temperatures as high as 160 K. The annealed samples with hole concentration p > 2 × 10 18 cm -3 show a positive Curie-Weiss temperature and open hysteresis curves, characteristic of a system with ferromagnetic correlations.

Low-dose aluminum and boron implants in 4H and 6H silicon carbide

Aluminum and boron p-type low-dose implants have been characterized in 4H- and 6H-SiC for anneals from 1300 °C to 1600 °C. In contrast to previous studies of heavily doped p-type layers, here we study more lightly doped layers for use as active regions in high-voltage power devices. Activation rates of the implanted ions, depth profiles from secondary mass ion spectroscopy, and surface roughness data using atomic force microscopy are presented as a function of anneal temperature. The temperature dependence of the free hole density and hole mobility are characterized with Hall effect measurements. For 1600 °C anneals, usable device quality p-type layers are obtained for both SiC polytypes and implant species. For anneals at or below ∼1500 °C, the implanted layers have much higher sheet resistivity due to the presence of unannealed compensating defects. These layers are not device quality. B-implanted layers have higher mobility, while activation of implanted Al is much higher and more uniform. Therefore, boron and aluminum have different advantages and disadvantages as p-type implants in SiC.

Hole diffusion at the recombination junction of thin film tandem solar cells and its effect on the illuminated current–voltage characteristic

Computer simulation of experimental current density–voltage (J–V) and quantum efficiency characteristics of thin film p1-i1-n1-p2 structures and of double junction solar cells (p1-i1-n1-p2-i2-n2), has been used to understand the hole transport mechanisms near the np “tunnel” junction between two subcells of a multijunction structure. Two different types of p layers at the junction have been studied: (i) hydrogenated microcrystalline silicon (μc-Si:H) and (ii) hydrogenated amorphous silicon carbide (a-SiC:H). There is a striking difference between the experimental J–V characteristics for the p1-i1-n1-p2 structures, with case (i) having a fairly high fill factor (FF) and conversion efficiency (η), as against a very low FF and η in case (ii). Although the difference is much smaller for double junction cells employing these two types of materials as the p layer at the junction, the fill factor of the cell employing μc-Si:H is about 8% higher. Analysis of transport properties as a function of position by computer modeling reveals that the main difference in behavior between the two cases is due to the much higher free hole population in the p layer at the junction when it is microcrystalline; which in turn, is a direct consequence of the lower activation energy for this case. We also learn that not only tunneling and the electric field in the bottom subcell, but also diffusion, plays a major role in pushing the holes produced in it by the incident light towards the recombination layer at the junction; and thereby helps improve cell performance, especially its fill factor. We conclude that the p layer at the junction should have a high free hole density (low activation energy in the device), to attain an overall high fill factor and conversion efficiency. Another interesting inference is the fact that tunneling as transport mechanism for holes towards the junction is more important when the p layer at the junction is a-SiC:H than when it is microcrystalline, while diffusion plays a more prominent role in propelling holes towards the junction in the latter case.

Si and C δ-doping of GaAs grown by metal organic vapour phase epitaxy for fabrication of nipi doping superlattices

The growth conditions for Si and C δ-doped nipi doping superlattices in GaAs have been optimised at the growth temperature of 630°C. We found that the Si δ-doping concentration can be significantly changed by δ-doping time over the range of 10 12–10 13 cm −2 at the optimised gas flow velocity. The similar range of the free hole density in C δ-doped GaAs has also been obtained simply by varying the TMAl flow rate during the δ-doping step. The full compensation of free electron and hole density in the Si and C δ-doped nipis can be achieved by choosing proper Si δ-doping time and TMAl flow rate. Growth of one Si and C δ-doped nipi in GaAs was demonstrated. Apart from the well-known effect of photo-excitation intensity on the effective band gap energy, the time-resolved photoluminescence reveals that the photoluminescence peak wavelength significantly increases in the relaxation process of the photo-excited nipi doping superlattice.

Electrical activation of carbon δ-doped (Al,Ga)As grown by metalorganic vapour-phase epitaxy

Carbon δ-doped (Al,Ga)As was grown by metalorganic vapour-phase epitaxy using trimethylaluminium (TMAl) or trimethylgallium (TMGa) as a doping precursor. The best C δ-doped Al0.3Ga0.7As has a peak hole density of 1.6 × 1019 (1.4 × 1019 for GaAs) cm−3 with a full hole profile width at half maximum of 85Å(84Åfor GaAs). For C δ-doped Al0.3Ga0.7 As grown at 630°C, the use of TMGa as a doping precursor leads to both the sheet C atom density and the free hole density increasing with an increase in the total TMGa moles introduced during a δ-doping step. As a result, the electrical activation remains almost constant with the change of TMGa moles supplied. The sheet C atom density always increases with increasing supply of TMAl, but approaches its maximum value at an amount of TMAl of 6.4 × 10−7mol. The electrical activation reduces from > 90% to < 10% when the supply of TMAl increases from 2.1 × 10−7 to 8 × 10−7mol. Regardless of the doping precursors, the hole density weakly decreases and the C atom density significantly increases with increasing growth temperature. Low growth temperatures are required for high electrical activation. Using optimised growth conditions, C δ-doped pipi doping superlattices with different average hole densities are fabricated to obtain C bulk-doped-like layers.

Lattice vibrations and phonon-plasmon coupling in Raman spectra of p-typeIn0.53Ga0.47As

Raman spectra of p-type ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As with doping levels ranging from p=2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{19}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}3}$ have been investigated. Analysis of the Raman line shape and its dependence on the free-hole density demonstrates four-mode behavior of optical phonons. Based on the oscillator strength and Faust-Henry factors of the optical phonons determined from the Raman data, and lattice-dynamic calculations of the phonon dispersion relations in ordered structures of ${\mathrm{InGaAs}}_{2}$, we show a relationship between the observed four-mode behavior and short-range order phase-separation effects.

Doping and compensation phenomena of Ag in CdTe

In this work the addition of Ag to p type CdTe is systematically studied by electrical characterisation combined with controlled ionimplantation. In one set of experiments radioactive Cd isotopes were implanted, acting during annealing as host atoms, but later transmuting according to the nuclear decay series into Ag acceptors. In this way a doping efficiency close to 100% is obtained. Furthermore Ag ions were implanted into the back side of CdTe wafers to investigate in situ the compensation process. The decrease in free hole density is caused by the formation of complexes involving acceptor impurities and interstitial Ag atoms.

Substrate bias effect on the capture kinetics of random telegraph signals in submicron p-channel silicon metal–oxide–semiconductor transistors

This letter investigates the effect of the substrate bias on the capture kinetics of random telegraph signals in submicron silicon p-channel metal–oxide–semiconductor transistors. A strong dependence of the capture time constant τc on the transverse electric field is observed. As a result, τc∼exp(−Ap) is observed experimentally, which is much stronger than the 1/p dependence predicted by simple Shockley–Read–Hall theory, whereby p is the surface density of free holes. The observations are explained tentatively by considering a field-dependent hole capture cross section. The latter may result from quantization effects induced by the transverse field in the two-dimensional inversion layer.

Melt-grown p-type GaAs with iron doping

Optical and electrical properties are described for bulk GaAs, grown from a melt doped with iron to create FeGa deep acceptors in a sufficient amount (exceeding the EL2 defect concentration) to make high-resistivity p-type rather than semi-insulating material. Both iron photoionization and EL2+ photoneutralization contribute to the near-infrared optical absorption. This made it possible to deduce the concentrations (NAi and NAn) of ionized and lattice-neutral iron, and the ratio (NAi/NAn). Temperature dependent measurements of dc electrical transport yielded quantities such as the free hole density, and hence the Fermi energy, for the 290–420 K range. This information combined with (NAi/NAn) led to a determination of the iron acceptor’s free energy εA(T): about 0.46 eV above the valence band at 300 K, and ∼40 meV closer at 420 K. The temperature dependence of εA for iron is shown to differ from εv, εc, midgap, or the free energy for CrGa acceptors in GaAs.