Deep Impurity Levels Research Articles

Detailed analysis of quasi-ohmic contacts to high resistive GaAs:Cr structures

Proper choice of a contact material leads to the reduction of leakage current, an increase of X-ray penetration coefficient when so-called transparent contacts are used, and an increase of the Signal-to-Noise ratio. This work is dedicated to the investigation of quasi-ohmic contact behaviour in the “Me-GaAs:Cr-Me” system. AuGe contact was made by means of electron-beam deposition as a metallization layer (Me) to form the “Me-GaAs:Cr-Me” system. Chromium compensated GaAs samples of different thicknesses in the range of 250–1000 μm were tested. The investigation was carried out under various temperature conditions. The results of current-voltage dependence measurement give an overview of the current transport model in such structures and allow us to calculate the concentration of deep-level impurities.

Doping sites observation of deep-level impurities in hyperdoped silicon via high-resolution transmission electron microscopy

The direct observation of the doping sites of deep-level impurities in hyperdoped silicon is essential to determine the impurity diffusion, distribution, and precipitation via different hyperdoping processes or thermal annealing conditions. This is crucial to clarify the functioning of a mechanism, which can improve the impurity electroactivity. However, this observation, which is usually performed by using high-resolution transmission electron microscopy (HRTEM), is limited by some restrictions in the microscopic imaging conditions, such as the microscope resolution, the defocus of the objective lens, the specimen thickness, and the projected orientation of the specimen (i.e. the zone axis). Therefore, to eliminate these restrictions, conventional field-emission-gun HRTEM images in combination with image processing (deconvolution) were investigated in order to obtain structure images with atomic resolution. Both the substitutional and the interstitial doping sites of oxygen, chalcogen (sulfur and selenium), and transition metal (titanium and cobalt) impurities in silicon were determined by image contrast analysis. Finally, the favorable conditions for the direct observation of doping sites were defined.

CMOS-Compatible Catalyst for MacEtch: Titanium Nitride-Assisted Chemical Etching in Vapor phase for High Aspect Ratio Silicon Nanostructures

Metal-assisted chemical etching (MacEtch) is an emerging anisotropic chemical etching technique that has been used to fabricate high aspect ratio semiconductor micro- and nanostructures. Despite its advantages in unparalleled anisotropy, simplicity, versatility, and damage-free nature, the adaptation of MacEtch for silicon (Si)-based electronic device fabrication process is hindered by the use of a gold (Au)-based metal catalyst, as Au is a detrimental deep-level impurity in Si. In this report, for the first time, we demonstrate CMOS-compatible titanium nitride (TiN)-based MacEtch of Si by establishing a true vapor-phase (VP) MacEtch approach in order to overcome TiN-MacEtch-specific challenges. Whereas inverse-MacEtch is observed using conventional liquid phase MacEtch because of the limited mass transport from the strong adhesion between TiN and Si, the true VP etch leads to forward MacEtch and produces Si nanowire arrays by engraving the TiN mesh pattern in Si. The etch rate as a function of etch temperature, solution concentration, TiN dimension, and thickness is systematically characterized to uncover the underlying nature of MacEtching using this new catalyst. VP MacEtch represents a significant step toward scalability of this disruptive technology because of the high controllability of gas phase reaction dynamics. TiN-MacEtch may also have direct implications in embedded TiN-based plasmonic semiconductor structures for photonic applications.

Effects of NH3 flow on structural and optical properties of ZnO films grown by atmospheric-pressure chemical vapor deposition

Structural and optical properties of zinc oxide (ZnO) films doped with nitrogen (N) grown on r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using zinc, water, and ammonia (NH3) as source materials have been studied. The small amount of NH3 flow was found to be effective for improving the film orientation and surface morphology. Photoluminescence (PL) spectra of the as-grown undoped and N-doped ZnO films were composed of a near-band-edge emission with two peaks at ~376 and ~386 nm and a green band (GB) emission with a peak at ~540 nm. PL and PL excitation (PLE) measurements for the as-grown N-doped films grown at the different NH3 flow rates indicated the possibility of the contribution of the N and/or hydrogen (H) related defect to both the appearance of the peak at ~386 nm and the excitation process of the GB emission. The comparison between the PL and photoacoustic (PA) spectra showed that the N atom occupying oxygen site (NO) or its complex defect is the most conceivable candidate for the non-radiative center responsible for the intra-band-gap PA absorption. After post annealing under the different conditions, a red band (RB) emission appeared at ~720 nm besides the GB emission. Both the RB and GB emissions observed for the annealed films were found to be effectively excited by the band-to-band (inter-band-gap) and free exciton transitions. Regardless of the difference in annealing condition, all the PLE spectra for the RB emissions showed a weak excitation band extending from 460 nm to 680 nm, suggesting the formation of the deep impurity level probably due to the NO acceptor. It was also confirmed by X-ray photoelectron spectroscopy that the NO deep acceptors were formed in the films, but passivated by the H atoms and/or compensated by the H related donors and/or by the nitrogen molecule substituting oxygen site [(N2)O] double donors.

Mechanism research of negative resistance oscillations characteristics of the silicon magnetic sensitive transistor with long base region

A silicon magnetic sensitive transistor (SMST) with the negative resistance oscillation phenomenon is presented in this paper. The SMST of cubic structure is composed of three regions and three electrodes (E, C and B). Two of the regions (collector region and base region) are designed on the top surface of the SMST, and the emitter region is designed at the bottom of the SMST. Using microelectromechanical system (MEMS) technology, the chip is fabricated on [Formula: see text] orientation p-type silicon (near intrinsic) wafer and packaged on printed circuit board (PCB). When collector voltage (V[Formula: see text]) and the base injecting current (I[Formula: see text]) are a certain value, the experimental results show that the collector current (I[Formula: see text]) attains negative resistance oscillation phenomenon and it is influenced by the external magnetic field (B) and temperature (T). Based on the effect of deep-level impurities on the carrier net recombination rate, theoretical analysis demonstrates that the deep-level impurities are the main factors of the appearance for oscillations phenomenon.

The Mechanism of Superfast Switching of Avalanche <inline-formula> <tex-math notation="LaTeX">${S}$ </tex-math> </inline-formula>-Diodes Based on GaAs Doped With Cr and Fe

The results of theoretical and experimental investigation of charge carrier transport in avalanche S-diodes based on $\pi $ - $\nu $ -n and $\pi $ -n structures are presented. High-ohmic layers of the diodes were made by diffusion of deep chromium and iron acceptors into n-GaAs. It is shown that recharge of the deep acceptors in the avalanche regime should lead to expansion of the space charge region into the $\pi $ -layer and formation of step-type current–voltage characteristics rather than the S-type. It has been found experimentally that the switching of the S-diode is superfast (the time of switching is less than the transient time of the carriers through the active region). The obtained results are in contradiction with the earlier proposed mechanism of deep-level recharging. Thus, this mechanism has been revised. The comparison of the obtained results with the literature data allows one to find the only mechanism of superfast switching, which is associated with generation of collapsing field domains due to the Gunn effect under the avalanche breakdown condition. According to the experiment, the switching time of S-diodes depends on the applied voltage and the type of the deep-level impurity. The S-diodes can operate in relaxation oscillator and sharper circuits. The use of the S-diodes in a sharper circuit with a moderate voltage rate of 1011 V/s allows generating the voltage pulses with amplitude of 700 V and a rising edge of 250 ps at a load of $50~\Omega $ .

Enhancement of the deep-level emission and its chemical origin in hexagonal boron nitride

Defect-related deep-level emissions at ∼4 eV from hexagonal boron nitride (h-BN) have been extensively investigated during the past decades. Although the emission has often been associated with deep level impurities such as carbon and oxygen, the structural and chemical origins of the emission center have not yet been identified. Here, we perform systematic photoluminescence measurements and quantitative trace impurity analysis of variously heat-treated h-BN samples with different deep-level emission intensities. In contrast to the common belief, no positive correlation between the impurity (carbon and oxygen) concentration and the deep-level emission intensity was found. We also demonstrate that the intensity of the deep-level emission is significantly enhanced by high-temperature heating (∼1900 °C) under an N2 environment and subsequent post-annealing at ∼900 °C. These results suggest that intrinsic defects created under N2-rich conditions are responsible for the deep-level emission. Furthermore, we found that besides the well-recognized zero-phonon line (ZPL) at ∼4.1 eV, additional ZPLs are induced in the near ultraviolet (NUV) region by appropriate annealing of the N2-treated h-BN sample. The results of temperature-dependent and time-resolved PL measurements on these NUV emission bands are given and discussed.

On the insulator-to-metal transition in titanium-implanted silicon

Hyperdoped silicon with deep level impurities has attracted much research interest due to its promising optical and electrical properties. In this work, single crystalline silicon supersaturated with titanium is fabricated by ion implantation followed by both pulsed laser melting and flash lamp annealing. The decrease of sheet resistance with increasing Ti concentration is attributed to a surface morphology effect due to the formation of cellular breakdown at the surface and the percolation conduction at high Ti concentration is responsible for the metallic-like conductivity. The insulator-to-metal transition does not happen. However, the doping effect of Ti incorporation at low concentration is not excluded, which might be responsible for the sub-bandgap optical absorption reported in literature.

Structural and optical properties of ZnO co-doped with Co and Ni thin films deposited by pulse laser deposition technique

Thin films of Zn0.9 Co0.1−xNixO (x = 0.0, 0.02, 0.04, 0.06, 0.08, 0.1) series were deposited by Pulsed Laser deposition technique (PLD) on a single crystal (100) silicon substrate. The structure of the films was confirmed as single phase hexagonal wurtzite structure by X-ray diffraction technique (XRD). The highest intensity peak was found at (002) plane. The surface morphology of thin films studied by Scanning Electron Microscope (SEM) depicted the high grain growth with definite boundaries. The room temperature resistivity measurements were carried out by four-point Probe methods which showed the semiconducting behavior of the thin films. Magnetic properties characterized by Vibrating Sample Magnetometer (VSM) at room temperature, exhibited the weak ferromagnetic or superparamagnetic behavior due to the alignment of the spins in the same direction around the oxygen vacancy and Co/Ni ions. Optical properties were observed by Spectroscopic Ellipsometry. The thickness of the films lied in 200–265 nm range and the bandgap energy was found to decrease from 3.09 to 3.03 eV confirming the inclusion of Ni a deep level impurity.

Defect physics in intermediate-band materials: Insights from an optimized hybrid functional

Despite the efforts to implement the idea of a deep level impurity intermediate band (IB) into bulk solar cell materials, a breakthrough in efficiency increase has not yet been achieved. Taking Sn-doped ${\mathrm{CuGaS}}_{2}$ as an example, we investigate the problem here from the perspective of defect physics, considering all possible charge states of the dopant and its interaction with native defects. Using an optimized hybrid functional, we find that ${\mathrm{Sn}}_{\mathrm{Ga}}$ has not only a donor-type (+/0), but also an acceptor-type ($0/\ensuremath{-}$) charge transition level. We estimate the probability of the optical transition of an electron from/to the neutral defect to/from the conduction-band edge to be about equal, therefore, the lifetimes of the excited carriers are probably quite short, limiting the enhancement of the photocurrent. In addition, we find that doping with ${\mathrm{Sn}}_{\mathrm{Ga}}$ leads to the spontaneous formation of the intrinsic acceptor ${\mathrm{Cu}}_{\mathrm{Ga}}$ defects which passivate the donor ${\mathrm{Sn}}_{\mathrm{Ga}}$ and pin the Fermi level to a position (1.4 eV above the valence-band edge) where both defects are ionized. As a result, the possibility of absorption in the middle of the visible range gets lost. These two recombination and passivation mechanisms appear to be quite likely the case for other donors and other similar host materials as well, explaining some of the experimental bottlenecks with IB solar cells based on deep level impurities.

Correlated electron-hole mechanism for molecular doping in organic semiconductors

The electronic and optical properties of the paradigmatic F4TCNQ-doped pentacene in the low-doping limit are investigated by a combination of state-of-the-art many-body \emph{ab initio} methods accounting for environmental screening effects, and a carefully parametrized model Hamiltonian. We demonstrate that while the acceptor level lies very deep in the gap, the inclusion of electron-hole interactions strongly stabilizes dopant-semiconductor charge transfer states and, together with spin statistics and structural relaxation effects, rationalize the possibility for room-temperature dopant ionization. Our findings reconcile available experimental data, shedding light on the partial vs. full charge transfer scenario discussed in the literature, and question the relevance of the standard classification in shallow or deep impurity levels prevailing for inorganic semiconductors.

Doping of Graphene Nanoribbons via Functional Group Edge Modification.

We report the on-surface synthesis of 7-armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups. The CN groups are attached to the GNR backbone by modifying the 7-AGNR precursor. Whereas many of these groups survive the on-surface synthesis, the reaction process causes the cleavage of some CN from the ribbon backbone and the on-surface cycloisomerization of few nitriles onto pyridine rings. Scanning tunneling spectroscopy and density functional theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon and introducing deep impurity levels associated with the nitrogen electron lone pairs.

Mechanism on M (M[dbnd]Ni, Mo, Ni[sbnd]Mo) as deep level impurity reducing the TCR of Si-rich Cr[sbnd]Si resistive films

CrSi–M (MNi, Mo, NiMo) resistive films were prepared by magnetron sputtering technique at the same process conditions. Experimental results shows that the metal M can reduce the temperature coefficient of resistance (TCR) of Si-rich CrSi resistive films, and that the resistive films follow the sequence of CrSiMo (6.34 at%) < CrSiNi (9.97 at%) < CrSiNi (6.08 at%)Mo (2.47 at%) according to TCR tending to zero. XRD analysis reveals that CrSi2 is the main conductive phase in the prepared CrSi–M resistive films. In order to explore the reasons that M can reduce the TCR of Si-rich CrSi resistive films, first principles was used to study the influence of M on the performance of CrSi2 from state density and band structure. The first-principles study on M-doped CrSi2 shows that M may exist in CrSi2 semiconductor in the form of deep level impurity, and an energy band model on both Ni and Mo-doped CrSi2 is constructed to reflect the results of first-principles simulation. Based on the established model, a mechanism is proposed that deep level impurities as electron traps capturing electrons and the formation of tight-binding excitons can suppress the non-equilibrium hot carriers against being contributed to the conductivity of CrSi2 semiconductor. As a result, the TCR of Si-rich CrSi resistive films is reduced due to the doping of M as deep level impurity.

The effect of shallow vs. deep level doping on the performance of thermoelectric materials

It is well known that the efficiency of a good thermoelectric material should be optimized with respect to doping concentration. However, much less attention has been paid to the optimization of the dopant's energy level. Thermoelectric materials doped with shallow levels may experience a dramatic reduction in their figures of merit at high temperatures due to the excitation of minority carriers that reduces the Seebeck coefficient and increases bipolar heat conduction. Doping with deep level impurities can delay the excitation of minority carriers as it requires a higher temperature to ionize all dopants. We find through modeling that, depending on the material type and temperature range of operation, different impurity levels (shallow or deep) will be desired to optimize the efficiency of a thermoelectric material. For different materials, we further clarify where the most preferable position of the impurity level within the bandgap falls. Our research provides insight on why different dopants often affect thermoelectric transport properties differently and directions in searching for the most appropriate dopants for a thermoelectric material in order to maximize the device efficiency.

Effect of Fe doping on the electronic structure, optical and electrical properties of ZnO compound: Ab initio insights

By means of first-Principe method within TB-mBJ approach and Boltzmann transport equations, the electronic structure, optical and electrical properties of zinc oxide doped with Iron are investigated. Both calculated band gap and lattice parameters of pure ZnO are found to be in reasonable agreement with the experimental measurements. Furthermore, the doping ZnO with Fe leads to the appearance of some deep impurity levels in the band gap arisen from Fe-3d states. Indeed, these deep level donors enhance the infrared and visible-light absorption. Besides, we notice that the electrical conductivity is enhanced while the carrier mobility is decreased after Fe doping.

Excitons and fundamental transport properties of diamond under photo‐injection

AbstractAbstractauthoren Exciton diffusivity and carrier mobility are key features in determining transport properties of a material. The optimum values required for high‐performance devices are expected to be realized in undoped intrinsic materials. Diamond, one of the emergent high‐mobility materials, has a wide band gap suited for use in power electronics and optoelectronics. For many years, however, mobility measurement for diamond has been limited by carrier freezing at deep impurity levels. Recently, attempts at utilizing photoexcited carriers in undoped diamond have been made in a wide temperature range and various excitation schemes. A series of investigations demonstrated a wealth of intriguing observations such as dominant role of excitons and emergence of low‐temperature anomaly in momentum relaxation. This article highlights recent experimental advances and progresses in understanding transport properties of photoexcited intrinsic diamond.

Predicting the electronic and optical properties of IB metals doped monoclinic BiVO4: First principle calculations

The effect of doping IB metals (Cu, Ag, and Au) on the photocatalytic activity of monoclinic Bismuth vanadate (BiVO4) was investigated by predicting their electronic and optical properties based on the first‐principle calculation. For the Cu‐doped system, the formation of shallow acceptor level will narrow the band gap and trap the electrons which may enhance the separation of photo‐induced carriers. And the doping of Ag has little effect on the electronic structure. However, the doping of Au will introduce a deep localized impurity level which should serve as both charge traps and recombination centers, and hinder the separation of photo‐induced carriers. With incorporation of Cu, the mobility of charge carriers is enhanced due to the smaller effective mass, but the Ag and Au doping would not improve the transfer of charges. In summary, only the doping of Cu has positive effect on improving the photocatalytic activity of monoclinic BiVO4. © 2015 Wiley Periodicals, Inc.

Optical properties of layered III–VI semiconductor γ-InSe:M (M=Mn, Fe, Co, Ni)

Indium selenide belongs to layered III–VI semiconductors with highly anisotropic optical and electronic properties. Energy gap of 1.32eV makes this material very attractive for solar energy conversion. We investigated the influence of 1% 3-d transition metals M=Mn, Fe, Co, Ni, used as dopants, on energy levels of InSe:M in the range 1.4–6.5eV and especially in the range of energy gap <1.4eV by means of ellipsometric measurements. It was concluded that at ambient temperature foregoing dopants, all divalent, with 4s2 valent electrons, in the similar way influenced on blue-shift of energy levels in valent zone, but did not influence on the fundamental energy gap. Photoluminescence measurements confirmed blue-shift of the valent zone energy levels and an existence of deep impurity levels.

Low-temperature galvanomagnetic studies of nominally undoped germanium subjected to intrinsic photoexcitation

The results of studying the heating of charge carriers by an electric field in nominally undoped Ge (with impurity concentrations of (N a + N g ) ≤ 5 × 1013 cm–3) subjected to interband illumination are reported. It is necessary in this situation to take into account two types of free charge carriers. In the case of such generation, the relation between the concentrations of electrons and holes depends to a large extent on the value of the electric field since this field differently affects the recombination coefficients of charge carriers and gives rise to new effects. The results of experimental studies of the conductivity σ and the Hall constant R H in n-Ge and p-Ge at T = 4.2 K and at different intensities of intrinsic photoexcitation are reported. A model of interband recombination, which takes into account deep-level impurity centers, is suggested for explanation of the results.

Nonlinear optical absorption in crystals with deep impurities: II. Band–impurity phototransitions involving free electrons

We propose a new mechanism for transitions of electrons from deep impurity levels to the conduction band under the action of light with photon energy Et smaller than ionization energy Et of the impurity center. Energy deficiency δ = Et–ℏω is covered by the kinetic energy of free electrons in the conduction band.