Thermal Ionization Energy Research Articles

Observation of negative-U centers in 6H silicon carbide

Two negative-U centers in 6H SiC have been observed and characterized using capacitance transient techniques. These two defects give rise to one acceptor level (−/0) and one donor level (+/0) each in the band gap. The donor and the acceptor level have inverted ordering, i.e., the thermal ionization energy of the acceptor level is larger than that of the donor level. Direct evidence for the inverted ordering of the acceptor and donor levels and temperature dependence studies of the electron capture cross sections of the acceptor levels are presented.

Electrical Properties of Oxygen Doped GaN Grown by Metalorganic Vapor Phase Epitaxy

AbstractDeliberate oxygen doping of GaN grown by MOVPE has been studied. The electron concentration increased as the square root of the oxygen partial pressure. Oxygen is a shallow donor with a thermal ionization energy of 27 ±2 meV. A compensation ratio of Θ = 0.3-0.4 was determined from Hall effect measurements. The formation energy of ON of EF = 1.3 eV, determined from the experimental data, is lower than the theoretically predicted value.

The DX Center in GaAsP Alloys

Using Deep Level Transient Spectroscopy (DLTS), we have investigated the properties of the DX center in GaAsP for the whole range of alloy composition x. We have determined the variation of the defect characteristics (thermal ionization energy E i , barrier for electron capture B, and energy level location E DX ) versus x. From the relationship that exists between E i , B and E DX , and between B and Δ, the energy difference between the L band and the bottom of the conduction band, we deduce that electron emission and capture occur from and to the DX center via the L band in the same fashion as in GaAlAs alloys. A good fit of the variation of E i and B versus x is obtained in a model where the DX level is a donor state associated with the L band which is 200 meV deep, like in GaAlAs, as a result of intervalley mixing.

Semi-Empirical Calculations of Hole Polarons in MgO and KNbO3 Crystals

The semi-empirical quantum chemical INDO method has been used for cluster and large unit cell calculations of hole polarons bound to a cation vacancy in highly ionic MgO and partly covalent perovskite KNbO 3 . In both cases a hole is well localized on an oxygen atom displaced towards the vacancy. The calculated optical and thermal ionization energies for V - and V 0 centers are in excellent agreement with experimental data for MgO. In KNbO 3 we predict the existence of one-site and two-site (molecular) polarons with close absorption energies (1 e V). The relevant experimental data are discussed.

Barrier height determination for n-type 4H-SiC schottky contacts made using various metals

We have studied Schottky barrier contacts to n-type 4H-SiC with Cr, Mo, Ta, W, Au, and Ni. We have focused on effects of the metal work function, measurement technique and interface behavior on the Schottky barrier heights (SBHs). The contacts were prepared by metal deposition via high frequency cathodic sputtering on chemical vapor epitaxially grown epitaxial layers with low residual doping (2 × 1015 cm−3). Prior to deposition on Si-terminated surfaces, they were in-situ cleaned by Ar-ion sputtering for 5 and 10 min, respectively. The contacts have been characterized by means of current-voltage, capacitance-voltage (C-V), and internal photoemission (IPE) methods at room temperature. With deep level transient spectroscopy, interface deep electron traps have been detected with thermal ionization energies of 0.68, 0.77, and 1.04 eV, respectively. These traps have been attributed to structural defects formed during epitaxial growth termination. The diodes have relatively low reverse leakage current, but the ideality factor is larger than one. The SBHs have been determined from C-V and IPE measurements. It has been shown that the C-V data may contain errors resulting in a low SBH if electron deep traps are present in the interface region. In general, the SBH of various metals are influenced by the metal work function and also by the semiconductor surface preparation. The interface homogeneity is an important characteristic of the Schottky contacts, which may be improved by an optimized ion sputtering prior to metal deposition. We have considered the SBHs determined by IPE measurements as most reliable.

Thermal ionization energy of Si and Mg in AlGaN

Compositional dependence of activation energies of Mg and also Si in AlGaN was obtained by the Hall effect measurements at various temperatures. Activation energies in AlGaN with AlN molar fraction less than 0.4 were characterized both theoretically and experimentally. The experimental result fits quite well for the Si donor with the hydrogen atom like model while that of Mg acceptor deviates downward from the model. In the p-type AlGaN, room temperature hole concentration of 1×1017cm−3 was achieved in AlxGa1−xN with x=0.33.

Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.

Electron paramagnetic resonance of conduction-band electrons in silicon

The g value of conduction-band electrons in silicon was properly determined by using electron paramagnetic resonance. A linear empirical relationship was first found between the g values and the thermal ionization energies of several well-known group-V substitutional shallow donors in silicon. An extrapolation of the empirical relation to zero ionization energy predicted the g value of conduction-band (CB) electrons, ${\mathrm{g}}_{\mathrm{CB}}$=1.9995, which is slightly but definitely different from that of conduction electrons in the donor-impurity band of degenerate n-type silicon; although both g values have been tacitly accepted to be identical for nearly four decades. The prediction was directly verified by measuring the g value of CB electrons created either by thermal emission from shallow donors in phosphorus-doped silicon at T=125 K and by above-band-gap optical excitation in high-purity p-type silicon at T=3.5 K; the measured g value in both experiments was precisely ${\mathrm{g}}_{\mathrm{CB}}$=1.9995(1). The empirical relation is still not theoretically explained.

Theoretical evidence for efficient p-type doping of GaN using beryllium

Ab initio calculations predict that Be is a shallow acceptor in GaN. Its thermal ionization energy is 0.06 eV in wurtzite GaN; the level is valence resonant in the zincblende phase. Be incorporation is severely limited by the formation of Be3N2. It is shown, however, that co-incorporation with reactive species can enhance the solubility. H-assisted incorporation should lead to high doping levels in metal-organic-chemical-vapor deposition growth after post-growth annealing at about 850 K. Be-O co-incorporation produces high Be and O concentrations at molecular beam epitaxy growth temperatures.

Deep level defects in electron-irradiated 4H SiC epitaxial layers

Deep level defects in electron-irradiated 4H SiC epitaxial layers grown by chemical vapor deposition were studied using deep level transient spectroscopy. The measurements performed on electron-irradiated p+n junctions in the temperature range 100–750 K revealed several electron traps and one hole trap with thermal ionization energies ranging from 0.35 to 1.65 eV. Most of these defects were already observed at a dose of irradiation as low as ≈5×1013 cm−2. Dose dependence and annealing behavior of the defects were investigated. For two of these electron traps, the electron capture cross section was measured. From the temperature dependence studies, the capture cross section of these two defects are shown to be temperature independent.

Deep electronic levels in high-pressure Bridgman Cd 1− xZn xTe

The behavior of deep electronic levels was studied as a function of Zn concentration in CdZnTe crystals grown by the high-pressure Bridgman technique using thermoelectric effect spectroscopy. A significant increase of the thermal ionization energies of hole traps was observed with the increasing Zn content of the ternary compound. The effect explains the stronger hole trapping and the resulting much shorter hole lifetime usually observed in CdZnTe as compared to CdTe. The behavior also suggests increased carrier recombination and explains the strong deterioration of electron collection in detectors fabricated from CdZnTe of high Zn concentration.

Charge-state effects of deep centres in semiconductors on non-radiative capture of carriers by multiphonon processes

A set of simplified analytical expressions for carrier capture coefficients, including quantatively the charge-dependent effect, have been obtained for easy physical examination and comparison with experiments. The temperature-related charge-state-dependent factorF(T) thus calculated could be used to present more accurately the nature and magnitude of the charge state of a trap centre. The ranges of values ofF(T) valid for attractive, repulsive and neutral centres are also obtained. In addition, we show that the thermal ionization energy for theB centre in GaAs is a function of temperature. The importance of the data of capture cross-section at low temperatures in determining the charge state and characteristic of a deep centre is also manifested. Both the absolute magnitude and the temperature-dependent behaviour of the calculated capture cross-section are well-supported by the very good fits to the experimental electron cross-sections forA andB centres in GaAs reported by Lang [7] and Wang et al. [22] and for Cu centre in Ge reported by Zhdanova and Kalashnikov [23].

Optical and electrical characterization of boron impurities in silicon carbide grown by physical vapor transport

Undoped SiC crystals grown by physical vapor transport have been characterized by temperature dependent Hall effect and near infrared optical absorption measurements. Crystals with reduced nitrogen content were found to exhibit p-type conductivity with carrier concentrations in the 5×1014–1×1016 cm−3 range at room temperature. The Fermi level position determined from Hall effect measurements at elevated temperatures was 0.35 eV above valence band. The primary acceptor-type impurity was identified as substitutional boron with total concentration of uncompensated acceptors in the 1×1017–5×1018 range. This interpretation was confirmed by near infrared absorption spectra, which were dominated by a broad photoionization band with a threshold at 0.7 eV and a maximum at 1.75 eV. The shape of the band was fitted, and the thermal ionization energy of the defect was found to be in the 0.3–0.4 eV range. A correlation between the photoionization band intensity, and the uncompensated boron content was used to determine the value of maximum optical cross section of boron photoionization band, which was 4.17×10−17 cm2. In addition to photoionization band, boron-containing samples exhibited set of narrow absorption lines near the fundamental absorption edge. Based on correlation with boron content and line position in different SiC polytypes, these lines were identified as due to excitons bound to neutral boron acceptors.

Surface Recombination in Semiconductors

We propose two general criteria for a surface defect state to act as an efficient, nonradiative recombination center. The first is that the thermal ionization energy should not deviate from the mid-gap energy by more than the relaxation energy of the defect, In this case the activation energy for the recombination is given by the barrier for the capture of the first carrier, whereas the second carrier is captured athermally. The second citerion is related to the position of the average dangling bond energy relative to the band edges. If, as in the cases of InP or InAs, it is located close to a band edge, a low surface recombination velocity is expected. However a much faster recombination is predicated and experimentally observed in the materials with the average dangling bond energy located close to the mid-gap. The relevance of these criteria for the novel wide-gap optoelectronic materials is discussed.

Deep-center oxygen-related photoluminescence in GaAs doped with dimethylaluminum methoxide during organometallic vapor phase epitaxy

Infrared oxygen-related photoluminescence measurements of GaAs epilayers grown by organometallic vapor phase epitaxy (OMVPE) and intentionally doped with dimethylaluminum methoxide (DMALMO) are reported. The photoluminescence emissions at 1.08, 0.95, 0.81, and 0.62 eV are detected. The centers responsible for the emissions at 0.81 and 0.62 eV are tentatively attributed to Al–O–Ga and Al–O–Al, respectively, based on correlation with layer growth conditions. The growth parameters lead to information on the number of nearest-neighbor aluminum associated with oxygen. The Al–O–Ga center responsible for the 0.81 emission is also studied for AlxGa1−xAs(x=0.05–0.9) layers grown by OMVPE without DMALMO. The 0.81 eV emission is attributed to the transition from the conduction band to the Al–O–Ga center. The center shows the lattice relaxation exemplified by the Franck–Condon shift of 0.15 eV. The thermal ionization energy of this acceptor is determined to be 0.56 eV above the valence band.

Optical-absorption study of synthetic pyrite FeS2 single crystals

The optical-absorption study of synthetic pyrite FeS2 single crystals prepared by the chemical-vapor-transport method is reported over a temperature range from 4.2 to 300 K. At lower temperatures several features which occur below the absorption edge are superposed on the absorption curve. The origin of the sharpest peak is identified as the transition between the top of valence band and Cr3+ trapping center through comparison of the results with that of the low-temperature photoconductivity and the photoelectron paramagnetic resonance measurements. The nature of other weaker features is discussed. The existence of these features prevents the determination of the band gap of FeS2 by fitting the optical-absorption data to the form of (αhν)∝(hν−E0)n. Therefore, the energy gap of FeS2 at low temperatures is estimated by adding the thermal ionization energy of the shallow acceptor states to the photoionization energy between the (SCl)2− states and the bottom of the conduction band. The band gap of various temperatures is determined and its temperature dependence is analyzed by an empirical expression proposed by O’Donnell and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of Eg of the material are evaluated and discussed.

Photoconductivity study of synthetic FeS2 single crystals at low temperatures

The photoconductivity measurements of synthetic pyrite FeS2 single crystals at low temperatures are used to study the impurity and defect-related photoresponse peaks below the band edge. At temperatures below 100 K, several features on the low-energy side of the photoconductivity spectra are observed. By comparison of the results with those of the absorption and the photo-EPR measurements, the origin of the sharpest peak at 0.866 eV is identified. The nature of other weaker features is discussed. The existence of these features prevents us from determining the band gap of FeS2 by the Moss rule at low temperature. We propose that the energy gap of FeS2 at low temperatures may be determined by adding the thermal ionization energy of the shallow acceptor states to the photo-ionization energy between the (SCl)2- states and the bottom of the conduction band.

0.8-eV photoluminescence of GaAs grown by molecular-beam epitaxy at low temperatures.

We report deep-center photoluminescence of GaAs grown at low temperatures between 200 and 400 \ifmmode^\circ\else\textdegree\fi{}C by molecular-beam epitaxy. Changes of temperature and excitation intensity were used in addition to heat-treatment and photoluminescence-excitation measurements. The type of photoluminescence transition strongly depends on the growth temperature and the [${\mathrm{As}}_{4}$]/[Ga] ratio. The layers grown at 200--300 \ifmmode^\circ\else\textdegree\fi{}C show a dominant emission at 0.68 eV (EL2) and an emission at 1.1 eV related to a ${\mathit{V}}_{\mathrm{Ga}}$ center. The layers grown at 325--400 \ifmmode^\circ\else\textdegree\fi{}C show one transition at 0.75--0.81 eV (designated as the 0.8-eV emission). We attribute the 0.8-eV emission to the ${\mathrm{As}}_{\mathit{i}}$-${\mathit{V}}_{\mathrm{Ga}}$ center. The center shows the large lattice relaxation exemplified by the Franck-Condon shift of 0.34 eV. The thermal ionization energy is determined to be 0.36 eV. We discuss our assignment of the ${\mathrm{As}}_{\mathit{i}}$-${\mathit{V}}_{\mathrm{Ga}}$ center to the well-known EL6 center.

A study of the electrical properties controlled by residual acceptors in gallium antimonide

Carrier concentration and mobility were investigated by Hall measurements over a wide temperature range (7-700 K) in nominally undoped p-GaSb grown from the melt. The main parameters relating to levels controlling the conductivity and to scattering mechanisms limiting the mobility were derived by a self-consistent fit of carrier density and mobility versus temperature. In the temperature range 20-500 K an approach using two acceptor levels successfully explained the temperature variation of the hole density. A negligible influence of non-polar optical phonons resulted from the analysis of the high temperature mobility, whereas an impurity band conduction dominates the electric transport at low temperature. Photoluminescence spectra were measured in the interval 2-300 K to confirm the energy location of the main localized level: by considering a correction factor for the compensation effect, complete agreement between optical and thermal ionization energies was found.

Characteristics of electron traps in In0.5Ga0.5P generated by recombination enhanced defect reactions

We report the observation of a new type of intrinsic defect in n-In0.5Ga0.5P which can be generated by recombination enhanced defect reaction (REDR) mechanism. It is observed that the increases of the concentrations of this defect and of another native defect due to REDR have nearly linear time dependence before saturation. This observation and other experimental results suggest that the two observed defects are complex defects. Other electrical properties of these defects such as alloy broadening effect on the thermal ionization energy are also described.