Donor Activation Energy Research Articles

Nitrogen donors and deep levels in high-quality 4H–SiC epilayers grown by chemical vapor deposition

4H-SiC epilayers grown by chemical vapor deposition were characterized by Hall effect, admittance spectroscopy, low-temperature photoluminescence, and deep level transient spectroscopy (DLTS). The nitrogen (N) donor activation energies were estimated as 45–65 meV at hexagonal and 105–125 meV at cubic sites from Hall effect investigations in agreement with the data taken by admittance spectroscopy. In low-temperature photoluminescence, the N bound exciton peaks were dominant, however, free exciton peaks were also observed. DLTS measurements revealed a low concentration of electron traps (∼1013cm−3) for both samples grown on Si and C faces, indicating high-quality epilayers independent of the substrate polarity.

Semi-insulating GaAs layers grown by molecular-beam epitaxy

Temperature dependent conductivity and Hall-effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 420 °C and separated from the substrate. The layers exhibit semi-insulating properties with a room temperature resistivity of (6–7)×106 Ω cm. The electron Hall mobility is proportional to temperature as ∼T−1.1 and the room temperature value is 5900 cm2 V−1 s−1. From the (nHT−3/2) vs T−1 plot, a donor activation energy of 0.68 eV, different than in bulk semi-insulating GaAs, has been evaluated. A similar activation energy is observed in as-grown and annealed GaAs layers grown at 200–350 °C. This indicates that the high resistivity of GaAs grown at low temperatures might be explained by deep donor defects rather than As precipitates.

Photoconductivity in n-type beta -FeSi2 single crystals.

Photoconductivity in \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ single crystals was observed in the temperature range of 80--250 K. The energy gap, 0.89 eV at 85 K, and its temperature dependence were determined. The values of the average phonon energy, 55 meV, and the electron-phonon coupling parameter, S=2.75, were evaluated. The observed quenching of photoconductivity is explained in assuming the two-center model. The donor and acceptor activation energy of 70 and 120 meV, respectively, as well as the hole mobility due to lattice scattering were determined.

Shallow donors, metallic conductivity, and metal—insulator transition in n-type CuInSe2

The activation energy of the shallow donors in n-type CuInSe2 is calculated from the available data of n and μ versus T. The variation of this energy with electron concentration is analyzed taking into account the screening effect of the charge carriers. A critical concentration of ncb = 1.4 × 1017 cm−3 above which the Fermi level merges into the conduction band and another nc = 5.7 × 1017 cm−3 that defines Mott's metal—insulator transition, are estimated. These values, that set the lower and upper limits for metallic and variable range hopping conductions, respectively, seem to agree with reported experimental data.

Hopping conduction and its photoquenching in molecular beam epitaxial GaAs grown at low temperatures

As the growth temperature of molecular beam epitaxial GaAs is increased from 250 to 400°C, the dominant conduction changes from hopping conduction to band conduction with a donor activation energy of 0.65 eV. A 300°C grown layer is especially interesting because each conduction mechanism is dominant in a particular temperature range, hopping below 300K and band conduction above. Below 140K, the hopping conduction is greatly diminished (quenched) by irradiation with either infrared (hv≤1.12 eV) or 1.46 eV light, but then recovers above 140K with exactly the same thermal kinetics as are found for the famous EL2. Thus, the 0.65 eV donor, which is responsible for both the hopping and band conduction, is very similar to EL2, but not identical because of the different activation energy (0.65 eV vs 0.75 eV for EL2).

Excess dark current due to saw damage in semi-insulating GaAs

Logarithmic plots of current or carrier concentration vs inverse temperature in semi-insulating GaAs samples are nearly always well characterized by straight lines with a donor activation energy of 0.76±0.02 eV, as long as the surfaces are well polished or etched and the edges are cleaved. However, edges cut with common dicing saws produce an excess conductance which can dominate the bulk conductance at room temperature and below. This excess conductance is due to carrier hopping between defects in a surface layer of approximately 2000A thickness, which can be easily removed by etching. Some polished surfaces also exhibit this same effect.

Identification and activation energies of shallow donors in cubic SiC

Silicon carbide grown by the chemical vapor deposition process on silicon often contains high concentrations of nitrogen donors with a binding energy of 54 meV as determined from photoluminescence studies. In contrast, the conductivity in the same samples is dominated by a heavily compensated shallow donor with a binding energy below 20 meV. The conventional view is that these two donors are the same. The 54 meV donors usually are assumed to be isolated substitutional nitrogen in regions of low concentration while the <20 meV donors are believed to be nitrogen in regions of high concentration. The most convincing evidence for this identification is the continuum of binding energies from 50 to 15 meV for SiC as a function of donor concentration compiled from published results. Evidence for reassessing the conventional view is given in this article and several experiments supporting the conventional view are reconsidered. As a result of this reconsideration, we propose that the donors below 20 meV which dominate the conductivity are not neutral, substitutional nitrogen in high concentration but some other center. D− centers and donor complexes are possible origins of this donor. The D− center appears to have many of the characteristics required for satisfactory modeling of this material.

Annealing dynamics of molecular-beam epitaxial GaAs grown at 200 °C

By separating a 2-μm-thick molecular-beam-epitaxial GaAs layer grown at 200 °C from its 650-μm-thick substrate, we have been able to obtain accurate Hall-effect and conductivity data as functions of annealing temperature from 300 to 600 °C. At a measurement temperature of 300 K, analysis confirms that hopping conduction is much stronger than band conduction for all annealing temperatures. However, at higher measurement temperatures (up to 500 K), the band conduction becomes comparable, and a detailed analysis yields the donor and acceptor concentrations and the donor activation energy. Also, an independent absorption study yields the total and charged AsGa concentrations. Comparisons of all of these quantities as a function of annealing temperature TA show a new feature of the annealing dynamics, namely, that the dominant acceptor (probably VGa related) strongly decreases and then increases as TA is increased from 350 to 450 °C. Above 450 °C, ND, NA, and [AsGa] all decrease, as is known from previous studies.

Activation energy and hall-coefficient measurements in lightly dopedn-InP in the thermal freeze-out region

The direct-current resistivity, β, and Hall coefficient,R H, of lightly dopedn-type InP samples were measured at temperatures (T) down to 12K and magnetic fields up to 4.8 kG. A sharp exponential increase in β, asT was decreased, was observed for temperatures below 80 K. The Hall coefficient showed a similar trend,i.e. R H increased sharply asT was reduced below 80 K. This is attributed to the freeze-out of conduction electrons onto their donor sites. The donor activation energy,E d, calculated from the temperature dependence of the resistivity, was less than the theoretical prediction. An enhanced dielectric constant would be a possible candidate for such behaviour. The initial decrease in β (asT is reduced) recorded in the higher-temperature region is due to impurity scattering probably combined with lattice scattering.

Temperature dependence of electrical properties of 3C-SiC(1 1 1) heteroepitaxial films

The electrical properties of self-supported undoped 3C-SiC(1 1 1) layers, heteroepitaxially grown on Si(1 1 1) by chemical vapor deposition (CVD) using a SiH 4-CH 3Cl-H 2 gas mixture, have been investigated at temperatures between 15 and 300 K. All the epilayers were n-type, and the carrier density and Hall mobility were 2.1×10 16−2.8×10 17 cm -3 and 120–180 cm 2/V s at room temperature, respectively. The activation energies of the residual donors obtained from the temperature dependence of the carrier density were 9–14 meV, and 90% of the donors were compensated by residual acceptors. The Hall mobility has a maximum (∼360 cm 2/V s) at around 100 K, and is limited by acoustic phonon scattering above 200 K and by ionized impurity scattering below 80 K.

Observations on the electrical characterization of the heteroepitaxially grown cubic SiC

This paper deals with the electrical characterization of thin layers of cubic SiC, grown on (100) Si substrates. The resistivity and Hall coefficient for undoped SiC layers were measured between 10 K and 500 K. The influence of inhomogeneities on the electrical properties of the as-grown films has been established. The Hall data show a clear sign of a transition to impurity band conduction. The donor concentrations studied are in the “intermediate” range. The donor activation energy has been shown to decrease with increasing nitrogen concentration. The nitrogen concentration was measured by SIMS. The variation in nitrogen concentration is also seen in changes in the shape of the ESR spectrum. The presence of nitrogen in the intermediate concentration range is the most likely reason for the conflicting values reported for the donor ionization energy as measured by Hall and PL measurements.

Hall analysis of the electrical properties of n-type AlxGa1-xAs grown by MOVPE

Variable temperature Hall measurements were used to study the electrical properties of undoped and Se-doped AlxGa1-xAs (0 <x < 0.4) layers grown by metalorganic vapour phase epitaxy (MOVPE). It is shown that the deep donor activation energy measured in undoped AlGaAs exhibits a similar dependency upon composition as that reported for Si-doped AlGaAs grown by MBE. For AlxGa1-xAs, doping with selenium is found to reduce the activation energy from 66 meV (forn = 4.1 x 1016/cm3), to 9 meV (forn= 1.6 × 1018/cm3).

Characterization of Mbe GaAs Layers Grown at 200°C–300°C

ABSTRACTLattice parameter, RBS channeling, and T-dependent Hall effect and resistivity are measured in 5 μm GaAs layers MBE-grown at 200°C, 250°C and 305°C, and subsequently annealed. Perpendicular lattice parameter was increased by 0.15% for 200 °C, 0.10% for 250°C, and 0.05% for 305°C sample. No parallel mismatch was observed. In annealing up to 800°C, most recovery of lattice parameter occured at 350°C-450°C. For samples annealed at 500° C − 600°C, the conductivity activation energy was higher in the furnace annealed sample than in the RTA-annealed sample, indicating possible depth nonuniformity in furnace annealed sample. The 600°C RTA annealed sample indicated a donor activation energy of 0.61 eV. A 600°C furnace annealed sample indicated the deep donor concentration of 1.5×1018 cm−3. Maximum mobility in the 500–600°C annealed sample was 3000–3500 cm2/Vs, limited by the neutral impurity scattering and polar optical phonon scattering.

Physical properties of arsenic-doped tin oxide thin films

Transparent conducting arsenic-doped tin oxide films were fabricated on a soda glass substrate by a chemical vapour deposition technique with an oxidation of SnCl 2. At 0.15 wt.% of arsenic doping the minimum resistivity and the maximum carrier concentration were found to be 1.5 × 10 −6 ωm and 9.0 × 10 26 m −3 respectively. The direct band gap was found to be 3.87 eV, transmittance of the film was about 90% and the maximum figure of merit at wavelength of 600 nm was found to be 13.1 × 10 −3 ω −1. The variation of thermoelectric power (TEP) with temperature for different concentrations has also been studied. The variation of conductivity with temperature indicates two donor activation energies for the films. X-ray diffraction studies demonstrate that SnO 2/As films deposited at lower temperature have poor crystallinity, but when deposited at 673 K, the films are polycrystalline. Scanning electron microscopic studies of these films indicate that the mean grain size varies from 0.20t to 0.45 ωm. It was found that the uniformity of the films and the mean grain size increases with deposition temperature.

Radiative electronic transitions in ZnTe∶Al, Ga, in crystals

We report the results of studies on the concentration dependence of the cathodoluminescence of ZnTe:Al, Ga, In crystals in the temperature range 4.2–300 K. A band with maximum at ∼2.2 eV and half-width of the order of 70 MeV and a band with maximum at ∼2.36 eV and half-width ∼1 MeV appear in ZnTe:Al crystals at 4.2 K. In the temperature range 40–70 K we detected the structure of the first band such that the separation of the maxima of the resolved bands corresponded to the energy of an LO phonon. A band with maximum at ∼1.8 eV was found to appear in ZnTe:Ga crystals at 4.2 K and a band with maximum at ∼1.9 eV appeared in ZnTe:In crystals at that temperature. The donor activation energies were determined to be 0.11, 0.30, and 0.25 eV, respectively, for aluminum, gallium, and indium.

Temperature dependence of electrical properties of n- and p-type 3C-SiC

Electrical properties of unintentionally doped n-type and Al-doped p-type 3C-SiC layers, epitaxially grown on Si by chemical vapor deposition, have been investigated at temperatures between 10 and 1000 K. Activation energies of Al acceptors and residual donors obtained from the temperature dependence of carrier density are 160 and 18 meV, respectively. 40%–60% of Al acceptors in the p-type epilayers are compensated, and hole mobility is limited by acoustic phonon scattering above 300 K and by ionized impurity scattering below 250 K.

Electrical Properties of 3C-SiC and its application to FET

ABSTRACTElectrical and luminescent properties of nondoped, and N-doped n-type 3C-SiC layers epitaxially grown on Si(100) by chemical vapor deposition were studied. Nondoped n-type epilayers with carrier concentration of 1×1016cm−3 and the Hall mobility of 750cm2/Vs at room temperature have the activation energy of donors, Ed=2OmeV, which is different from that of the donors in the N-doped layers. The photoluminescence spectra of nondoped layers are different from those of N-doped ones. These results suggest that the donors in the unintentionally doped n-type 3C-SiC are not due to N impurities. 45–70 % of N-donors in the N-doped epilayers are compensated.Schottky-barrier and MOS-type field-effect transistors have been fabricated from 3C-SiC. The transistor operations of MESFETs and MOSFETs were studied at elevated temperatures up to 440°C. Transconductances of 1.7mS/mm and 0.15mS/mm for MESFET and 0.8 and 0.05mS/mm for MOSFET at room temperature and 440°C, respectively, were obtained. The drain currentvoltage characteristics of both the FETs at room temperature did not change in the least after heating up to 440°C in the air.

Te doping of GaAs and Al xGa 1− xAs using diethyltellurium in low pressure OMVPE

The incorporation of Te in Al x Ga 1− x As grown by low pressure OMVPE using diethyltellurium (DETe) has been studied as a function of Al composition ( X = 0−0.4), vapor phase V/III ratio, doping level (i.e., DETe vapor mole fraction), and growth temperature T s . Variable temperature Hall effect measurements ( T = 20−400 K) have shown that, for a given alloy composition X Al' the “donor activation energy”, (i.e., the slope of log ( 1 eR H ) versus 1 T ) is smaller in Te doped Al xGa 1−xAs than in Si doped AlGaAs (e.g., 18 versus 60 meV for X Al = 0.3). A maximum 300 K carrier concentration of n = 1×10 19 cm −3 was obtained in Te-doped GaAs. The 300 K carrier concentration n 300 in Te doped GaAs decreased with increasing vapor phase V/III ratio (e.g., n = 5.0×10 17 cm −3 at a V/III ratio of 20 compared with n = 1.5×10 17 cm −3 at a V/III ratio of 120). The value of itnin300 also decreased with increasing growth temperature (e.g., n 300 = 2.3×10 18 cm −3 at T s = 600° C and n 300 = 8×10 16 cm −3 at T s = 750° C). Trends in the Hall mobility, photoluminescence efficiency at 300 K, and surface morphology are presented, and the Te doping “memory effect” will be discussed.

Low-temperature optical absorption in AlxGa1-xAs grown by molecular-beam epitaxy.

A detailed study of optical absorption in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As alloy system is undertaken using optical transmission and photoluminescence data obtained at 3 K from molecular-beam epitaxial layers. Absorption coefficient spectra are calculated for the entire alloy composition range 0\ensuremath{\le}x\ensuremath{\le}1. The first experimental data concerning AlAs in the region of the ${\ensuremath{\Gamma}}_{15V}$-${\ensuremath{\Gamma}}_{1C}$ band gap are presented. The compositional dependence of threshold values of the absorption coefficient agrees with current theoretical predictions. Detailed excitonic structure is observed for the first time in the absorption spectra of ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As epilayers with x\ensuremath{\le}0.43. From this structure, free-exciton binding energies are determined. These energies display an x dependence qualitatively similar to that observed in donor activation energies in n-type ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As.

Doping effects and compositional grading in AlxGa1-xAs/GaAs heterojunction bipolar transistors

First-order analytical calculations were made for the energy-band diagrams for n-Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As/p-GaAs heterojunctions for x = 0.15, 0.3, and 0.5 employing different compositional gradings and doping densities specifically for heterojunction-bipolar-transistor (HBT)applications. In the calculations most recently determined, conduction-band discontinuity ΔE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</inf> of 65 percent of the bandgap difference ΔE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> between the Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As and GaAs, and the donor activation energies in n-Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As of 60 and 160 meV for x = 0.3 and 0.5, respectively, were used. The results show that the position of the heterojunction spike barrier, and the depth and width of the notch in the conduction-band edge for a compositionally abrupt heterointerface depend on the respective doping densities on the p and n sides of the heterojunction. Also, for an abrupt heterointerface the difference in barrier heights for electron and hole injections varies between ΔE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> and ΔE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</inf> (the valence-band discontinuity), depending on the doping densities and the applied bias, and is not necessarily the generally accepted value of ΔE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</inf> . Analytical expressions and curves were obtained to estimate the minimum compositional grading L for eliminating the spike barrier and the notch as a function of the doping densities and the applied bias.