Acceptor Ionization Energy Research Articles

Photoemission spectroscopy of diluted Mn in and on solids

We have studied the electronic structure of the Mn-based diluted magnetic semiconductors and the ordered Mn surface alloys using photoemission spectroscopy and configuration interaction (CI) calculation. As for the Mn-based diluted magnetic semiconductors, the electronic-structure parameters obtained from the CI analysis show systematic variation with expected chemical trends. The CI picture gives a systematic description of the d–d absorption spectra, the exchange constants between the 3d spins and host band states, and the donor and acceptor ionization energies of the diluted magnetic semiconductors. The photoemission spectra of the ordered surface alloys CuMn/Cu(1 0 0) and NiMn/Ni(1 0 0) show correlation satellites similar to the impurity Mn atom in solids. It is shown that the CI analysis can describe the correlation satellites driven by the reduced dimensionality in the ordered surface alloy systems.

Pb(Zr,Ti)O3-GaN Heterostructures for RF MEMS Applications

Ferroelectric (Pb(Zr0.3Ti0.7)O3 or PZT(30/70))/GaN/Sapphire heterostructures were fabricated by the sol-gel process. The structure and composition of PZT, and its dielectric and ferroelectric properties in Pt/PZT/Ru or Pt/PZT/Pt, and Pt/PZT/GaN configurations were characterized to assess their feasibility as RF MEMS devices for eventual insertion in RF communication systems. The X-ray diffraction and pole figure analysis confirmed the stoichiometric, phase-pure PZT with (111) out-of-plane relationship on (0001) GaN. The secondary ion mass spectrometry (SIMS) depth profile indicated a stable interface with insignificant inter-diffusion of the Pb, Zr, or Ti elements into the GaN. The properties of PZT in Pt/PZT/Ru/GaN or MFM configuration were high capacitance density (C/A = 1.25 μF/cm2) and polarization (30 μC/cm2). In contrast, PZT in Pt/PZT/GaN or MFS configuration exhibited lower capacitance density (C/A = 0.35 μF/cm2) and asymmetrical hysteresis loops (polarization ∼4 μC/cm2). From the calculated spatial distribution of the electric field, E(x), and potential, V(x), which stem from all the charge densities within the entire MFS system, one may eventually design and fabricate PZT/GaN heterostructures in terms of controllable parameters (e.g., PZT phase and orientation, donor and acceptor ionization energies, and thickness and screening lengths of PZT and GaN), such that the depolarization field (Edepol) is minimized and the polarization is maximized in PZT.

Dependence of the ionization energy of shallow donors and acceptors in silicon on the host isotopic mass

A comparison of the infrared absorption spectra of isotopically pure ${}^{28}\mathrm{Si}$ and ${}^{30}\mathrm{Si}$ reveals small shifts in the transition energies for both the shallow donor phosphorus and the shallow acceptor boron. The impurity binding energies for both species are slightly larger in ${}^{30}\mathrm{Si}$ than in ${}^{28}\mathrm{Si}.$ A similar effect was earlier observed for the boron acceptor in ${}^{13}\mathrm{C}$ vs ${}^{12}\mathrm{C}$ diamond, and later explained as resulting from a change in the ground and excited state binding energies due to the dependence of the hole effective mass on the host isotopic composition. Here we show that the results for both donors and acceptors in Si can be explained by the same mechanism, with the additional inclusion of the isotopic dependence of the dielectric constant.

Electrical properties of nitrogen implanted GaSe single crystal

AbstractN‐implantation to GaSe single crystals was carried out perpendicular to c‐axis with ion beam of 6 × 1015 ions/cm2 dose having energy values 30 keV and 60 keV. Temperature dependent electrical conductivities and Hall mobilities of implanted samples were measured along the layer in the temperature range of 100‐320 K. It was observed that N‐implantation decreases the resistivity values down to 103 Ω‐cm depending on the annealing temperature, from the room temperature resistivity values of as‐grown samples lying in the range 106‐107 Ω‐cm. The temperature dependent conductivities exhibits two regions (100‐190 and 200‐320 K) with the activation energies of 234‐267 meV and 26‐74 meV, for the annealing temperatures of 500 and 700 °C, respectively. The temperature dependence of Hall mobility for the sample annealed at 500 °C shows abrupt increase and decrease as the ambient temperature increases. The analysis of the mobility‐temperature dependence in the studied temperature range showed that impurity scattering and lattice scattering mechanisms are effective at different temperature regions with high temperature exponent. Annealing of the samples at 700 °C shifted impurity scattering mechanism toward higher temperature regions. In order to obtain the information about the defect produced by N‐implantation, the carrier density was analyzed by using single donor‐single acceptor model. We found acceptor ionization energy as Ea = 450 meV, and acceptor and donor concentration as 1.3 × 1013 and Nd = 3.5 × 1010 cm−3, respectively. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Donor–acceptor pair luminescence in nitrogen-doped ZnO films grown on lattice-matched ScAlMgO 4 (0001) substrates

We have grown nitrogen-doped ZnO (ZnO:N) films by laser molecular-beam epitaxy. The use of lattice-matched ScAlMgO 4 substrates prevented the degradation of crystallinity induced by the nitrogen incorporation to the films. Despite this improvement, we have not obtained ZnO:N films which showed p-type conductivity. We studied the optical properties of these ZnO:N films. Donor–acceptor pair (DAP) luminescence was observed. The results indicate the formation of an acceptor state. The energy position of the DAP luminescence is lower than that reported by Look et al. [Appl. Phys. Lett. 81 (2002) 1830]. The DAP luminescence band shifts to lower energy with increasing nitrogen concentration. A photoluminescence recombination possibly due to the free-electron-to-acceptor (FA) transition was observed at temperatures higher than 40 K. The acceptor ionization energy was estimated from the energy position of the FA luminescence to be 266 meV.

Carrier leakage in InGaN quantum well light-emitting diodes emitting at 480 nm

Pulsed light–current characteristics of InGaN/GaN quantum well light-emitting diodes have been measured as a function of temperature, with sublinear behavior observed over the whole temperature range, 130–330 K. A distinctive temperature dependence is also noted where the light output, at a fixed current, initially increases with temperature, before reaching a maximum at 250 K and then decreases with subsequent increases in temperature. On the basis of a drift diffusion model, we can explain the sublinear light–current characteristics and the temperature dependence by the influence of the large acceptor ionization energy in Mg-doped GaN together with a triangular density of states function characteristic of localized states. Without the incorporation of localization effects, we are unable to reproduce the temperature dependence whilst maintaining emission at the observed wavelength. This highlights the importance of localization effects on device performance.

Liquid phase epitaxial growth and characterization of Ga 1− xMn xAs layers on (1 0 0) GaAs substrate

Ferromagnetic Ga 1− x Mn x As epilayers with Mn mole fraction in the range of x≈0.2–9.6% were grown successfully on semi-insulating (1 0 0) GaAs substrates using the liquid phase epitaxy technique. The ferromagnetic properties of these epilayers were investigated using superconducting quantum interference device at 5 K. The Curie temperature was determined to be 80 K. All the Ga 1− x Mn x As layers had p-type conductivity through Hall effect measurements. The measured hole concentration of Ga 1− x Mn x As layers decreases from 2.0×10 17 cm −3 to 9.8×10 15 cm −3 with increasing Mn mole fraction in the range of x≈0.2–9.6%. Cross-sectional selected area diffraction pattern of the GaMnAs films obtained through transmission electron microscopy measurement showed that the grown GaMnAs layer can be classified as a single crystal. Transitions involving shallow Mn acceptors were identified through photoluminescence measurements and the ionization energy of Mn acceptor was determined to be 104.6 meV.

Optical properties of Mn-doped GaAs layers grown on (100) GaAs substrate

Mn-doped Ga1−xMnxAs epilayers grown on semi-insulating (100) GaAs substrates using the liquid phase epitaxy technique were investigated using photoluminescence and Hall effect measurements from 20 to 300 K. Transitions involving shallow Mn acceptors were identified through photoluminescence measurements and the ionization energy of Mn acceptor was determined to be 104.6 meV which is in good agreement with values calculated from a hydrogenic hole model including sp–d exchange contribution. Also, it was concluded that Ga1−xMnxAs alloys were obtained from samples with low Mn concentration (below x≈1%) and these layers have a simple impurity band merging to the valence band.

Determination of the Nitrogen Acceptor Ionization Energy in Zinc Oxide by Photoluminescence Spectroscopy

ABSTRACTPhotoluminescence (PL) experiments performed on bulk ZnO crystals are used to establish the ionization energy of the substitutional nitrogen acceptor. The temperature dependence of the nitrogen-related electron-acceptor (e,A°) emission band has been monitored in as-grown single crystals. A lineshape analysis of this band is used to determine the acceptor ionization energy. The temperature variation of the ZnO band gap was included in our analysis and the low-temperature acceptor ionization energy for substitutional nitrogen at an oxygen site in ZnO was found to be EA = 209 ± 3 meV. Electron paramagnetic resonance and Hall-effect measurements were also used to characterize these bulk ZnO samples.

On the temperature coefficient of 4H-SiC BJT current gain

In this paper, the temperature coefficient of 4H-SiC NPN BJT current gain is studied by way of numerical simulations. In general, 4H-SiC NPN BJT would have a positive temperature coefficient (PTC) for the common emitter current gain if the acceptor ionization energy is smaller than 170 meV. Both PTC and negative temperature coefficient (NTC) can occur in 4H-SiC NPN BJT with an aluminum-doped base. High base doping concentration is required to obtain a wide range of current density with a NTC for current gain, especially when the electron lifetime in base is low. For a base doping concentration of 2.5×10 17 cm −3, the NTC for current gain is obtained for current density up to 300 A/cm 2, even when the electron lifetime is as low as 48 ns. The experimental results are also reported.

The Fermi level in diamond

Diamond is a wide-band-gap material with large donor and acceptorionization energies. In principle, at room temperature andbelow, the Fermi energy is pinned close to the donor oracceptor level, depending on which is present in the higherconcentration. In semiconductors with shallow donors and acceptors the equilibrium charge states of defects aredetermined by the position of the Fermi level. However, in aninsulating material like diamond we show that the calculatedposition of the Fermi level does not necessarily predict thecorrect charge state of a defect, and propose instead that the charge state is influenced by the proximity of the defect to adonor (or acceptor). Qualitatively this accounts for thedependence of the charge state on the concentration of isolatedsubstitutional nitrogen and also explains why many opticalcentres can be present in two different charge states in thesame diamond.

Luminescence properties of lithium-doped ZnS epitaxial layers grown by MOCVD

Lithium-doped p-type ZnS epitaxial layers have been grown on (0 0 1)-oriented GaAs substrates by low-pressure metalorganic chemical vapor deposition using tertiarybutyllithium as an acceptor dopant source. As-grown samples exhibited electrically high resistive n-type or p-type conductivity. However, all samples exhibited p-type conductivity after annealing in N 2 atmosphere at 550°C. The highest net acceptor concentration ( N A− N D) was estimated to be about 4.1×10 16 cm −3. Photoluminescence (PL) spectra of lithium-doped ZnS epitaxial layers with N A− N D<10 15 cm −3 were dominated by the radiative recombination of excitons bound to a lithium acceptor at 327.6 nm. On the other hand, PL spectra of lithium-doped ZnS epitaxial layers with N A− N D>10 15 cm −3 were dominated by free-to-acceptor emission with LO phonon replicas. From the peak energy of the FA emission, the lithium acceptor ionization energy was estimated to be about 196 meV.

Luminescent Properties of Sb Doped CdTe Grown by Hot-Wall Epitaxy

Sb-doped CdTe layers are grown by hot-wall epitaxy and their photoluminescence (PL) properties are investigated. The ionization energy of the Sb acceptor is estimated as 57 meV. Thermal annealing effects on the PL spectra are also investigated and the electronic state of Sb is found strongly affected by the presence of Cd vacancy.

Electrical characteristics of Mg-doped GaN activated with Ni catalysts

Electrical characteristics of Mg-doped p-GaN activated with Ni catalytic layers have been investigated by means of temperature-dependent Hall effect measurements. It has been revealed that the Ni layer on GaN enhances activation of the acceptor in the whole annealing temperature range. This enhancement is remarkable especially at temperatures below 500 °C and can be attributed to the catalytic effect for the hydrogen desorption. We have found that the donor concentrations in the samples activated with the Ni catalysts are also higher than those activated without Ni. This is probably due to the catalytic effect of Ni for the nitrogen desorption. In addition, it has been found that the ionization energies of the Mg acceptors in GaN activated with the Ni layer are decreased from 170 to 118 meV as the acceptor concentration increases.

Codoping method for the fabrication of low-resistivity wide band-gapsemiconductors in p-type GaN, p-type AlN and n-type diamond: prediction versusexperiment

We review our new valence control method of a co-doping for the fabrication oflow-resistivity p-type GaN, p-type AlN and n-type diamond. The co-dopingmethod is proposed based upon ab initio electronic structurecalculation in order to solve the uni-polarity and the compensation problemsin the wide band-gap semiconductors. In the co-doping method, we dope both theacceptors and donors at the same time by forming the meta-stableacceptor-donor-acceptor complexes for the p-type or donor-acceptor-donorcomplexes for the n-type under thermal non-equilibrium crystal growthconditions. We propose the following co-doping method to fabricate thelow-resistivity wide band-gap semiconductors; p-type GaN: [Si + 2 Mg(or Be)], [H + 2 Mg (or Be)], [O + 2 Mg (or Be)],p-type AlN: [O + 2 C] and n-type diamond: [B + 2 N],[H + S], [H + 2 P]. We compare our prediction of theco-doping method with the recent successful experiments to fabricate thelow-resistivity p-type GaN, p-type AlN and n-type diamond. We show that theco-doping method is the efficient and universal doping method by which toavoid carrier compensation with an increase of the solubility of the dopant,to increase the activation rate by decreasing the ionization energy ofacceptors and donors, and to increase the mobility of the carrier.

Mechanisms of incorporation of an antimony impurity into cadmium telluride crystals

The results of electrical studies of CdTe crystals grown by the Bridgman-Stockbarger method and doped with Sb impurity to concentrations of 1017–3×1019 cm−3 were considered. An analysis of the temperature dependences of the Hall coefficient, the charge-carrier mobility, and photoconductivity under the intrinsic-absorption excitation for various portions of the ingots made it possible to conclude that SbTe and SbCd centers and SbTeSbCd associations are introduced upon doping CdTe crystals with Sb impurity. The hole conduction in the doped crystals is controlled by A3 (SbTe) acceptors whose concentration is no higher than 5×1016 cm−3 and is much lower than the actual Sb concentration. The ionization energy of the A3 acceptors is 0.28±0.01 eV. Under nonequilibrium conditions, these acceptors act as attachment centers for holes (at high temperatures) and as slow-rate recombination centers for electrons (at low temperatures).

Calculations of acceptor ionization energies in GaN

The $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian and a model potential are used to deduce the acceptor ionization energies in GaN from a systematic study of the chemical trend in GaAs, GaP, and InP. The acceptors studied include Be, Mg, Ca, Zn, and Cd on the cation sites and C, Si, and Ge on the anion sites. Our calculated acceptor ionization energies are estimated to be accurate to better than 10% across the board. The ionization energies of C and Be (152 and 187 meV, respectively) in wurtzite GaN are found to be lower than that of Mg (224 meV). The C was found to behave like the hydrogenic acceptor in all systems and it has the smallest ionization energy among all the acceptors studied.

Amphoteric behavior of Ge dopants in In 0.5Ga 0.5P epilayers grown on GaAs(100) substrates

Ge-doped In 0.5Ga 0.5P epilayers grown on semi-insulating GaAs(100) substrates by the liquid phase epitaxy technique were investigated by using photoluminescence (PL) and Hall effect measurements. The Ge dopant existing in the In 0.5Ga 0.5P showed amphoteric behavior with a compensation ratio between 0.4 and 0.6. Exciton transitions involving the shallow Ge acceptor were identified by using PL measurements, and the ionization energy of the Ge acceptor was determined to be 47 meV, which was reasonable with a hydrogenic acceptor value. The results of Hall effect measurements showed that the ionization energy of the Ge shallow donor varied from 6 to 10 meV increasing the carrier concentrations.

Ionization energy of copper in Hg0.8Cd0.2Te crystals under conditions of light and intermediate doping

Resistivity and the Hall effect were studied in copper-doped p-Hg 0.8 Cd 0.2 Te crystals in the temperature range of 4.2–125 K and the range of Cu concentrations from 2.6×1015 to 2×1018 cm−3. It is shown that the conventional method for determining the ionization energy of impurities from the slope of the dependence R H (T) is inapplicable in this case. In order to obtain the correct results, it is necessary to take into account the structure of the impurity band and the screening of the impurity charge with free charge carriers. A simplified model of the impurity band is suggested; this model makes it possible to calculate the ionization energy of acceptors under conditions of light doping and a small degree of compensation. This approach is used to find that ionization energy of copper depends only slightly on the copper concentration at T=0 and is equal to E A =7.6 meV for an isolated acceptor, which coincides with the theoretical value. At finite temperatures, the ionization energy of acceptors decreases appreciably as a result of screening.

Doping of Al xGa 1− xN alloys

We have investigated the doping of Al x Ga 1- x N alloys on the basis of density–functional–pseudopotential calculations for native point defects and dopant impurities. We have identified two possible causes for the experimentally observed decrease in n-type conductivity for x>0.4: (1) in the case of doping with oxygen, a DX transition that converts the shallow donor into a deep level; and (2) compensation by cation vacancies. For p-type doping, the reduction in doping efficiency with increasing x is attributed to (1) compensation by nitrogen vacancies; and (2) an increase in the Mg acceptor ionization energy. We also investigated a number of alternative acceptor impurities; none, however, exhibit overall characteristics superior to those of Mg. Only beryllium emerged as a possible candidate, though it may be hampered by compensation due to Be interstitial donors.