Concentration Of Deep Centers Research Articles

INFLUENCE OF UNIAXIAL ELASTIC DEFORMATION ON MOBILITY OF CARRIERS OF THE CURRENT IN CRYSTALS n − Si AND n −Ge IN THE PRESENCE OF DEEP ENERGETIC LEVELS

An influence of uniaxial elastic deformation on change of mobility of carriers of a current in crystals n − Si with a deep level 0,17 c E − eV and n −Ge with a deep energetic level 0,2 c E − eV is investigated. During this concentration of deep centers in crystals n − Si and n −Ge features of dependences 0 f (X ) μ = μ at different temperatures those, that and in relation to clean crystals n − Si and n −Ge without deep levels in the conditions of mainly phonon dissipation is showed.

Study of the current-voltage characteristic of the n-CdS/p-CdTe heterostructure depending on temperature

Current-voltage characteristics of the n-CdS/p-CdTe heterostructure are studied at different temperatures. It is established that the forward portion of the current-voltage characteristic of these structures at low voltages (as high as 0.5 V) is described by the exponential dependence, while at high voltages (as high as 2.6 V), there is a portion of sublinear growth of the current with voltage. Experimental results are interpreted based on the theory of the effect of injection depletion. It is shown that the product of mobility of majority carriers by the concentration of deep centers increases as the temperature is increased.

Testing metal-dielectric-semiconductor tunnel structures on p-silicon as nuclear particle detectors

Metal-dielectric-semiconductor (MDS) structures with aluminum nitride (AlN) as a tunnel dielectric based on high-ohmic p-type silicon substrates have been studied. The samples were characterized with respect to the charge collection efficiency and energy resolution on probing with 5.4-MeV α particles. In addition, the nature of noises and the state of the AlN-p-Si interface were investigated. It is established that the parameters of these MDS structures as radiation detectors are close to those of widely used Schottky-barrier detectors based on n-Si (Au-n-Si). A decrease in the concentration of deep centers at the AlN-p-Si interface allows the proposed MDS structures to compete successfully with n-Si based detectors, which is due to a higher purity of the initial material.

Deep level generation in nitrogen-doped float-zoned silicon

In nitrogen-doped Float-Zone (FZ) silicon, nitrogen does not show any electrical activity in as-grown state. However, deep centers are known to emerge after annealing at relatively high temperatures like 900 or 1000°C. In the present work it was found that in FZ samples of relatively high initial resistivity (greater than 1000Ωcm, p- and n- type) the generation of deep centers is well pronounced after annealing at a lower temperature (680°C). In p-type samples, deep donors with an energy level above the midgap are produced and the material is converted into n-type of an extremely high-resistivity. In n-type samples, deep acceptors (with an energy level also above the midgap) are generated. The deep center concentration is on the order of 1013cm−3. A possible origin of the deep centers is interaction of a fast-diffusing interstitial nitrogen species with other impurity centers.

Effects of Annealing on the Electrical Properties of Nitrogen-Doped Float-Zoned Silicon

The nitrogen impurity, in nitrogen-doped Float-Zone (FZ) silicon, is known to show no electrical activity in as-grown state, but to produce deep centers after annealing at relatively high temperatures like 900 or 1000{degree sign}C. In the present work it was found that in FZ samples of relatively high initial resistivity (greater than 1000 Ohm.cm, p- and n- type) the generation of deep centers is well pronounced after annealing at a lower temperature (680{degree sign}C). In p-type samples, deep donors are produced, and the material is converted into n-type of an extremely high-resistivity. Another striking feature of this material is an extremely low apparent Hall mobility indicating to a well-pronounced non-uniformity in the carrier distribution. In n-type samples, deep acceptors are generated. The deep center concentration is on the order of 2x1013 cm-3. A possible identification of the deep centers is substitutional nitrogen.

Stability of deep centers in 4H-SiC epitaxial layers during thermal annealing

N -type epitaxial 4H-SiC layers grown by hot-wall chemical vapor deposition were investigated with regard to deep centers by capacitance-voltage measurements and deep level transient spectroscopy (DLTS). The DLTS spectra revealed that the concentrations of deep centers were reduced by one order of magnitude by annealing at 1700°C, compared to those in an as-grown material. The Z1∕2 center with an energy level of 0.59±0.03eV and the EH6∕7 center with an energy level of 1.66±0.11eV below the conduction band edge are annealed out at a temperature of 1700°C or higher.

Measurements of deep trap concentration in diodes with a high Schottky barrier by deep-level transient spectroscopy

The effect of the Schottky barrier height, energy position of deep traps, deep and shallow defect concentrations, temperature, and voltages applied to samples in the expression defining the deep center concentration in diodes with high Schottky barriers was studied by deep-level transient spectroscopy. Based on the results of the analysis, a method is suggested for determining the deep center concentration; this method is appropriate for both “normal” and “anomalous” deep traps.

Studies of the effect of proton irradiation on 6H-SiC pn junction properties

The influence of proton irradiation on current–voltage characteristics, N d– N a values and parameters of deep centres in 6H-SiC pn junctions has been studied. The irradiation was carried out with 8-MeV protons with doses from 10 14 to 10 16 cm −2. Irradiation with a dose of 5.4×10 15 cm −2 resulted in a very high resistance of forward-biased pn structures, remaining high even after anneling to 500°C. It is suggested that proton irradiation reduces the holes lifetime and increases the concentration of deep centers, which leads to formation of an i-layer.

Scanning electron microscope studies of AlGaN films grown by organometallic vapor phase epitaxy

Microcathodoluminescence (MCL) and electron beam induced current (EBIC) studies were carried out on AlGaN films grown by organometallic vapor phase epitaxy (OMVPE) on sapphire and SiC. It is shown that both non-uniformities of distribution of deep recombination centers and of electron concentration can exist in AlGaN layers and in some cases these non-uniformities can be associated with the presence of certain types of defects. For example, regions adjacent to pores in AlGaN films are often depleted of deep centers one of which is the center responsible for yellow luminescence in AlGaN. Electron concentration is enhanced in hexagonal hillocks commonly observed for unoptimized growth conditions of AlGaN. There were also found non-uniformities in concentration of deep centers or in electron concentration that could not be related to particular morphological features or structural defects. Diffusion lengths measurements for various AlGaN films are also reported. The diffusion lengths are shown to be in the 0.3–0.5 μm range for most of the samples. In some of the studied AlGaN films it was found necessary to introduce two diffusion length values to describe the experimental EBIC distribution profiles. The longer diffusion length is found to increase in value for lower donor densities. Minority carriers lifetimes are found to be longer in AlGaN compared to GaN films.

Evaluation of active layer properties and charge collection efficiency of GaAs particle detectors

According to Ramo's theorem the charge collection efficiency of a particle detector is mainly influenced by the field distribution between the contacts of a Schottky diode. In semi-insulating GaAs material a space charge layer is formed due to deep levels needed for the compensation of acceptors. In this paper the deep levels and their influence on the distribution of the electric field is studied experimentally by different methods of electrical characterization. It is found that the electrical active concentration of the midgap donor of ∼10 15 cm −3 at an energy of 0.67 eV below the conduction band is only about one tenth of its total concentration of ∼16 16 cm −3 as measured by infrared absorption. The Schottky barrier leakage current is found to be responsible for the variation of the electrically active deep centers and it therefore influences the charge collection efficiency (c.c.e.). The c.c.e. turns out to be inversely proportional to the active concentration of deep centers. These results are supported by our modelling of the c.c.e.: Using the transport and the Poisson equation the electrical field distribution can be calculated through the coupling of the quasi-Fermi levels and the compensation mechanism. The model calculations of charge collection efficiencies for both alpha particles and protons are confirmed by the experimental results. The work is performed within the framework of the RD8 project.

Failure phenomena due to hydrogen migration in amorphous a-SiN x:H films

A novel model of degradation phenomena in amorphous silicon nitride films is presented in this paper. We argue that three center Si-H-Si bonds which have negative effective electron correlation energies are dominant deep trapping centers in the investigated films. A new mechanism of hydrogen migration stimulated by the capture of nonequilibrium holes is proposed. The migration of hydrogen results in the deep center concentration decrease and in the creation of local leakage channels. Experimental evidence is presented in support of the proposed model and suggestions are made about how to obtain silicon nitride films with enhanced stability.

Thermal stability of ZnSe epilayer grown by MOVPE

The thermal stability of ZnSe heteroepitaxial layers grown at 250° C by the atmospheric pressure MOVPE is much better than that of heteroepitaxial layers grown at 300° C by conventional reduced pressure (0.1 Torr) MOVPE. The thermal stability of epitaxial layers is related to the growth conditions, which would determine the degree of stoichiometry in the epitaxial layers. However, heat treatment higher than 600° C in hydrogen ambient easily generates a high concentration of deep centers and promotes incorporation of impurities from the surface by some chemical reaction of ZnSe with hydrogen irrespectively of the degree of stoichiometry. On the other hand, an introduction of Zn vapor in H2 has the effect of suppressing the generation of deep centers. It is strongly suggested that Zn vacancy is directly related to the origin of deep centers. A reduction of its concentration under controlled atmosphere during high-temperature treatment is required to keep the high crystalline quality. Also the heat treatment in nitrogen ambient does not generate the deep centers even at 657° C. Consequently, it is found that ZnSe epilayer grown at 250° C by the atmospheric pressure MOVPE is thermally stable at least up to about 700° C.

D X center analysis in Sn-doped AlGaAs layer of double heterostructures

Capacitance as well as photovoltage methods have been used to analyze deep centers in an n-type AlGaAs:Sn layer of double heterostructures. It is suggested that the trap with thermal activation energy equal to ΔE2=0.33±0.02 eV is associated with the L minimum of AlGaAs and could be interpreted as a DX center related to Sn. The observed changes of deep center concentration in double heterostructures are due to different Al contents within the depletion region.

The Characterization of Germanium and Silicon for Nuclear Radiation Detectors.

ABSTRACTSemiconductor nuclear radiation detectors require deep depletion depths (0.03–3.0 cm) and effective charge collection distances which are several times longer than these depletion depths. These requirements place stringent limitations on the net electrically active impurity concentration, and on the concentration of deep centers which can trap carriers generated by the incident nuclear radiation. This need for extremely pure material distinguishes the interests and efforts of the semiconductor detector community from the rest of the semiconductor community. This paper reviews the characterization of shallow-level, deep-level, neutral, and extended defects in germanium and silicon for nuclear radiation detectors. Photothermal ionization spectroscopy has been used extensively to identify the residual hydrogenic impurities in high-purity (∣NA–ND∣ ≈ 1010–1011 cm−3 ) germanium and silicon. Deep level transient spectroscopy has been effectively used to detect and identify deeper levels in high-purity germanium. Residual neutral defects are not necessarily passive: they may complex to form deep or shallow levels, they may precipitate, or they may act as nucleation sites for precipitation. The properties of extended defects (dislocations, lineage, inclusions, precipitates) and their effects on device performance are fundamentally less well understood, as the origin of the electrical activity of these defects is uncertain. It has been found in numerous instances that chemical interactions among defects are important even in these high-purity semiconductors.

Electronic Characterization of Heteroepitaxial Silicon‐on‐Sapphire by Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy was successfully applied to silicon‐on‐sapphire and it enables simultaneous determination of band structure, trapping centers, deep levels, the refractive index, and the film thickness. Studies of “as‐grown” and ion‐implanted films revealed a noticeable degree of amorphization manifested by 1.45 eV energy gap structure clearly resolved in addition to standard 1.1 eV structure of bulk Si. Three deep levels were identified, i.e., electron trap at , hole trap at , and deep centers at . It has been found that the degree of amorphization and concentration of deep centers in the layers depends on the thermal history of the SOS film.

Preparation of low-resistivity n-type ZnSe by organometallic chemical vapor deposition

Low-resistivity n-type ZnSe with p<0.05 W cm and n≳1017 cm−3 has been grown epitaxially on (100) GaAs substrates by a low-pressure low-temperature organometallic chemical vapor deposition process. Triethylaluminum is used as a dopant. The as-grown layers show a strong near-band-gap photoluminescence peak. The much weaker photoluminescence intensity at longer wavelength indicates that the concentration of deep centers is lower than in doped ZnSe prepared by other methods.

A NEW METHOD FOR DETERMINATION OF DEEP-LEVEL IMPURITY CENTERS IN SEMICONDUCTORS

It is pointed out that the conventional capacitance method and the Copeland method provide different results for the determination of impurity concentrations of semiconductor wafers in the presence of deep centers. On the basis of a discussion of this phenomenon, a simple method of determining the concentration and the energy level of deep centers is proposed and demonstrated. The method is checked by application to oxygen-doped n-type GaAs with a carrier concentration of 3.2×1015 cm−3. The result yields a value of 2.1×1016 cm−3 for the deep center concentration and a value of 0.57 eV below conduction band for the energy level of the deep centers, which are reasonable in comparison with other experimental data.

Determination of deep centers in silicon by thermally stimulated conductivity measurements

The technique of measuring thermally stimulated conductivity in reverse-biased junctions has been applied to silicon. It is shown that this provides a simple and effective method to determine energies as well as concentrations of deep centers directly in a junction device. In several n +- n- p + Si high voltage rectifiers, four different trapping centers were detected in the 20Ω-cm n-region of the junctions. These centers have energies from 0.2 to 0.4 eV, and concentrations between 4×10 11 and 3×10 12cm −3.

Green Electroluminescence from Gallium Phosphide Diodes near Room Temperature

Detailed spectral properties of green near-bandgap electroluminescence have been studied between ∼50° and ∼380°K in several GaP diodes containing different concentrations of nitrogen impurities. The observed evolution of the room-temperature green bands from the readily identifiable low-temperature bands and a comparison of the room-temperature spectrum with the intrinsic luminescence band calculated from the absorption-edge spectrum show that intrinsic transitions are of minor importance for these diodes near 300°K. The observed room-temperature green luminescence in lightly doped diodes is identified with the recombination of bound and free excitons induced by isoelectronic nitrogen impurities, whereas radiative recombination of free holes at neutral shallow donor centers predominates in heavily doped diodes. The intensity of the green luminescence generally increased more rapidly than the red Zn, O acceptor-donor pair luminescence near 300°K, both with increase in temperature and in diode current. We have observed green electroluminescence external efficiencies approaching 10−4 for exciton recombination at room temperature in exceptional diodes containing relatively low concentrations of deep centers (notably those with low oxygen content). The calculated rate constant for intrinsic transitions and the largest observed mean minority-carrier lifetimes together suggest that intrinsic green luminescence of efficiency approaching 10−4 might be observable at room temperature from diodes containing currently attainable minimal concentrations of nitrogen and oxygen impurities, if the luminescence is not confined to the junction depletion layer.