Concentration Of Acceptor Impurities Research Articles

Electric Noise in Field-Effect Transistors Based on ZnO:Li Films

The noise characteristics of field-effect transistors based on ZnO: Li films and obtained by the diffusion technology are studied. The results of an experimental study of the noise characteristics of the drain current are presented, namely, the Random Telegraph Signal and 1/f-noise. The spectral density of the drain current noises in the low-frequency range (10–5000 Hz) has a classical 1/f-dependence. It was found that at low concentrations of the acceptor impurity, 1/f noise is observed, and with an increase in the acceptor impurity concentration the Random Telegraph Signal prevails.

Hydration of acceptor-doped BaSnO3: Implications of the bound states of ionic defects

The implications of acceptor-bound ionic defects in the hydration of acceptor-doped cubic perovskite BaSnO3 are elucidated based on the thermogravimetric analysis of water uptake and the authors’ recent statistical theory. The oxides BaSn1–xScxO3–δ (0.10 ≤ x ≤ 0.37) and BaSn0.9Sc0.1–xInxO3–δ (x = 0.05, 0.10) were synthesized and experimentally studied. The thermogravimetric measurements showed very good hydration properties of BaSn1–xScxO3–δ: the low-temperature hydrogen concentration was close to the nominal dopant content for all doping levels. The obtained data pertaining to the dopant concentration effect on the hydration isobars are well described by the exploited theory that accounts for the trapping of protons and oxygen vacancies by acceptors. The experimental findings on the effect of scandium substitution with indium on the hydrogen dissolution in the BaSn0.9Sc0.1–xInxO3–δ oxides are also in good agreement with our theoretical predictions. In addition, we discuss the impact of trapping on the hydration enthalpy, entropy and Gibbs free energy of BaSnO3 doped with various acceptors, and establish the type and concentration of acceptor impurities which enhance or inhibit hydration. Our results testify to the pivotal role of the defects’ trapping by acceptors in the hydration of barium stannate, and demonstrate that this effect is important for understanding and prediction of the hydration properties of acceptor-doped perovskites.

Optimizing two and four-terminal CuGaSe2/CuInGaSe2 tandem solar cells for achieving high efficiencies

Solar cells based on CuInGaSe2 (CIGS) have attained the highest record efficiency (22.9%) among thin film solar cells. CIGS is formed by the appropriate mixing of CuInSe2 (CIS) which has a bandgap around 1 eV and CuGaSe2 (CGS) which has a bandgap around 1.7 eV. These bandgaps are close to the optimal values for two-junction tandem solar cells. Therefore, CGS on CIGS or CGS on CIS tandem cells should be attractive and viable to achieve high efficiency and cost-effective thin film solar cells. In this work, a simple unified analytical model is used to design two-terminal and four-terminal CGS/CIGS tandem solar cells, establishing a realistic value for the expected efficiency in each case. Short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency, are determined as a function of the CGS and CIGS acceptor impurity concentrations, back surface recombination velocities, minority carrier diffusion lengths, and absorber layer thickness. It is shown that realistic efficiencies over 30% can be achieved under one sun AM1.5 spectrum for both of the terminal configurations, but the optimum composition for the CIGS sub-cell should be different depending on the respective tandem cell configuration.

Structural and optical properties of heavily doped Al x Ga1 − x As1 − y P y :Mg alloys produced by metal-organic chemical vapor deposition

The high-resolution X-ray diffraction technique, Raman spectroscopy, and photoluminescence spectroscopy are used to study the structural, optical, and electron energy properties of epitaxial AlxGa1 − xAs1 − yPy:Mg alloy films grown by metal-organic chemical vapor deposition (MOCVD). It is shown that the introduction of a Mg impurity into the quaternary alloy provides high charge-carrier concentrations. A decrease in the growth temperature yields a decrease in the charge-carrier concentration in films doped with magnesium at a small gas-carrier flux of the acceptor impurity, whereas an increase in the flux results in an increase in the acceptor-impurity concentration, which is reflected in the character of the photoluminescence spectra.

Influence of the dopant on the contact formation to p+-type silicon

Abstract In this contribution we investigate the influence of the doping element on the contact formation to p+-type Si. Contacting B doped layers with Ag thick film paste leads to very few Ag crystallites at the contact interface and results in poor contact resistances [1] , [2] , [3] . Using Ag/Al thick film paste for contact formation, the contact is not only formed by Ag crystallites, but by diversely shaped Ag/Al contact spots and the contact resistance is reduced by more than one order of magnitude [2] . Al melting at the Si wafer surface forms Al doped rectangles on the Si wafer where the growth of Ag/Al/Pb spikes is enhanced. When contacting Al doped layers with Ag thick film paste a larger number of Ag crystallites is observed than for B doped layers. If the contact is formed with Ag/Al paste the number of Al-rich rectangles is enhanced and we detect higher doped areas under the contact spots. The contacts detected have an ellipsoidal, pyramidal or “L-formed” shape. We conclude that not only the acceptor impurity concentration under the contact area is crucial for the contact formation, but also the properties of the specific acceptor present.

Effect of reactive ion beam etching on the photoluminescence of CdTe epitaxial layers

We demonstrated the effect of reactive ion beam etching (RIBE) process on the PL properties of CdTe/sapphire metal organic vapor phase epitaxy layers. At optimum conditions, the RIBE attack does not make significant morphological changes but it results in an increase of the concentration of acceptor impurities. This was revealed by an increase of the overall photoluminescence (PL) intensity and, simultaneously, a decrease of the PL decay time, more important on the low energy side of PL spectrum due to the recombination of carriers in acceptor pairs.

The hanle effect in nonuniformly doped GaAs

Spin density distribution in the GaAs/AlGaAs heterostructure was studied under steady-state optical orientation conditions. A comprehensive analysis of the dynamics and relaxation processes responsible for the steady-state nonuniformity of spin orientation was made. The acceptor impurity concentrations in various regions of nonuniformly doped gallium arsenide were determined. The concentrations were derived from the spin relaxation times measured using the optical orientation method.

Magnetic phase structure of Mn-doped III-V semiconductor quantum wells

We present a self-consistent calculation of magnetic phase structure in modulation-doped dilute magnetic III-V semiconductor quantum wells. The spin-split subband structure is calculated at finite temperature and magnetic-field effects on the carrier-induced magnetism are examined in a weak-field regime. The exchange correlation of free carriers is shown to enhance the ferromagnetic tendency. And the temperature ${T}_{\mathrm{th}},$ below which the system spontaneously becomes fully spin polarized, increases as the doping concentration ${N}_{a}$ in nonmagnetic barriers increases. Hysteresis loop of the magnetic quantum well predicts that the remnant magnetization is enhanced as one increases the modulation-doped acceptor impurity concentration.

Violet GaN based light emitting diodes fabricated by metal organics vapour phase epitaxy

For almost 2 decades, p-doping of GaN was not feasible. Since the ionisation energy of any acceptor species is >200 meV, the hole concentration obtained by p-doping is only one hundredth of the acceptor impurity concentration. Mg has been so far the most typical dopant used for p-type GaN. In metal organics vapour phase epitaxy (MOVPE), the precursor was the bismethylcyclopentadienyl Mg, (MeCp) 2Mg. However, two severe drawbacks must be overcome. The Mg precursor and ammonia react in the vapour phase to give solid particles. In addition, H is incorporated during the growth process, therefore, a post growth annealing is required. The Mg doped GaN samples studied were grown by MOVPE on (0001) oriented sapphire substrates. With proper design of the growth chamber and thermal annealing, doping densities up to 2×10 18 cm −3 have been reached. Photoluminescence (PL) and photocapacitance data reveal that in addition to the shallow acceptors, deep states are most likely related to Mg complexes. n-Doping is straightforward. Si is easily introduced via silane and allows a free carrier concentration up to 10 19 cm −3 to be reached. Henceforth p–n junctions and light emitting diodes were achievable.

Thin oxide thickness extrapolation from capacitance-voltage measurements

Five oxide-thickness extrapolation algorithms, all based on the same model (metal gate, negligible interface traps, no quantum effects), are compared to determine their accuracy. Three sets of parameters are used: (acceptor impurity concentration, oxide thickness, and temperature): (10/sup 16/ cm/sup -3/, 250 /spl Aring/, 300 K), (5/spl times/10/sup 17/ Cm/sup -3/, 250 /spl Aring/, 300 K), and (5/spl times/10/sup 17/ cm/sup -3/, 50 /spl Aring/, 150 K). Demonstration examples show that a new extrapolation method, which includes Fermi-Dirac statistics, gives the most accurate results, while the widely-used C/sub o//spl sime/C/sub g/ (measured at the power supply voltage) is the least accurate. The effect of polycrystalline silicon gate is also illustrated.

Optical quenching of bound excitons in CdTe and Cd1−xZnxTe alloys: A technique to measure copper concentration

A noncontact, nondestructive optical technique with potential to measure the acceptor impurity concentrations in bulk CdTe and Cd1−xZnxTe alloys is described here. The technique, called optical quenching, uses an infrared light source in conjunction with the above-band-gap excitation beam used in standard photoluminescence spectroscopy. The quenching, or decrease, of acceptor-bound exciton emission is related to the incident infrared beam flux. The rate at which the infrared photons ionize neutral acceptors can be used to determine relative concentrations of the substitutional centers. We have applied this technique to the copper acceptor-bound exciton in CdTe and Cd1−xZnxTe bulk samples. The 1.064 μm output from a Nd:YAG laser provided near-resonant absorption with the copper acceptor level. Relative copper concentrations in the range from 4×1014 to 1×1016 cm−3 were obtained. These values agree reasonably well with sample comparisons based on atomic absorption data.

Characterization of Fe-doped semi-insulating InP by low temperature and room temperature spatially resolved photoluminescence

Scanning photoluminescence has been used to characterize four 2 inch (100) wafers cut from the same Fe-doped semi-insulating LEC 〈111〉 InP ingot. The photoluminescence mappings have been done both at room temperature and at low ( ̃ 10 K) temperature. In the latter case, scannings have been performed with an optical resolution of 20 μm at two different energies: 1.417 and 1.380 eV. The highest energy corresponds to the band-to-band photoluminescence emission, while the lowest one is associated with the band-to-acceptor (in our case Zn and C) transition. Higher optical resolution ( ̃1 μm) has been also achieved at low temperature but without any energy selection. The photoluminescence data reveal substrate non-homogeneities which have been attributed to variation of Fe concentration. The images of the 2 inch wafers show clear growth striations as well as a central growth facet. At the microscopic scale, dislocations corresponding to bright photoluminescence dots have been identified. Higher dislocation density has been found along some of the growth striations. Areas exhibiting higher acceptor impurity concentration have been also detected. Finally, the resistivity mapping of the slice cut from the ingot under investigation show a direct correlation with the photoluminescence results.

Determination of donor and acceptor impurity concentrations in n-InP and n-GaAs

The Hall mobility of semiconducting materials is generally computed without accounting for the Hall factor,r H. It is shown that for low magnetic fields this may lead to considerable errors in the calculation of the impurity concentrations,N D andN A, in both n-GaAs and n-InP of reasonable purity. Tables allowing the determination ofN D andN A under low magnetic field conditions are computed at room and liquid nitrogen temperatures. They account forr H and for the inelastic nature of collisions with polar optical phonons without assuming Matthiessen's rule.

Efficiency of solar cells with a light-trapping design

The pair generation rate, ideal characteristics and conversion efficiency of a solar cell with a specific thin-film design discussed previously are calculated and compared with those of a conventional design chosen as standard. Various factors affecting the efficiency are examined in order to find parameters that will improve the efficiency relative to that of the standard cell. It is found that for an n- on p-substrate Si thin film of thickness 2.5 mu m based on the present design, the efficiency can be improved significantly relative to that of the standard cell by exploiting the back surface field effect and by raising the acceptor impurity concentration to a level comparable to the donor impurity concentration.

Defect Models for Sintering and Densification of Al2O3: Ti and Al2O3: Zr

A defect model proposed to explain the effect of titanium doping on the rate of sintering of Al/sub 2/O/sub 3/ is revised to fit the oxidizing conditions of the experiments. The model accounts for the observed change in sintering rate by a change from rate limitation by ions to rate limitation by electrons, but requires the presence of an unusually large concentration of acceptor impurities in the material. Models similar to the ones originally proposed account for the rate of densification of Al/sub 2/O/sub 3/:Zr by hot-pressing in vacuo, provided it is extended by including electronics defects.

The Electrical Properties of Zinc Selenide Heat-Treated in Controlled Partial Pressures of Constituent Elements

ZnSe single crystals, some of which had been previously heat-treated in molten Zn or doped with Cu, were heat-treated in Zn or Se vapor. The Hall effect and the electrical conductivity were measured and examined as a function of the vapor pressure. Conduction after heat treatment in Se vapor is considered to be dominated by Zn vacancies, Se interstitials, or associated defects containing one of these, while the high n-type conductivity observed after heat treatment in Zn vapor is dominated by Se vacancies or Zn interstitials. With decrease of the acceptor impurity concentration, the transition vapor pressure from low to high conductivity shifts from a high Zn vapor pressure to a low one corresponding to the total pressure minimum.

Calculated Energy Levels and Optical Absorption inn-Type Si Accumulation Layers at Low Temperature

Self-consistent sub-band splittings and inter-sub-band optical matrix elements are calculated for $n$-type accumulation layers at temperatures low enough that the bulk carriers are frozen out. The energy splittings are sensitive to the concentration of acceptor impurities in the surface space-charge layer.

Shallow-donor ionization energies in the II-VI compounds

Hall data on ZnSe, CdS, CdSe, and CdTe have been analyzed to determine the shallow-donor ionization energies ${E}_{d}$ and the total donor and acceptor impurity concentrations ${N}_{d}$ and ${N}_{a}$, respectively. Lowering of the ${E}_{d}$ values owing to impurity concentration effects are noted even in the purest samples. When the data are analyzed using linear plots of ${E}_{d}$ vs ${({N}_{{d}^{+}})}^{\frac{1}{3}}$, the resulting values of the ionization energy for infinite dilution are in good agreement with optical data and with the hydrogenic model.

Investigations of Scattering and Trapping Processes of Carriers in Ultrapure Germanium Films

Films of about 100 μ thickness were grown by iodide-transport process in a closed tube, with the concentration of acceptor impurities below 1012 cm−3. Measurements were made on the samples, which had one natural surface of film growth and the other surface prepared by the usual polishing and etching procedure. The Hall coefficient and surface-conductivity measurements were made by a dc field-effect method in a temperature range between 100 and 300 K. Results of the measurements are expressed in terms of surface mobility and trapped charge curves. The scattering of electrons is a diffuse process for both conditions of the surface (natural-grown surface and treated surface of the film) and over the whole temperature range. The scattering of holes is diffuse on the treated surface and only partially diffuse on the natural surface of film. The probability of diffuse reflection for holes in this case is equal to 0.3–0.4 and does not depend on temperature. The dominant mechanism of the surface scattering is discussed for different cases. The trapped charge curves for both conditions of the surface correspond to the assumption of a continuous spectrum of surface states. The form of the spectrum is independent of temperature for the natural surface. For the treated surface it changes with temperature, owing to an adsorption process.