Hole Capture Cross Section Research Articles

Field effect passivation of high efficiency silicon solar cells

In this paper effective surface recombination velocities Seff at the rear SiSiO2 interface of the presently best one -sun silicon solar cell structure are calculated on the basis of measured oxide parameters. A new cell design is proposed allowing for a control of the surface space charge region by a gate voltage. It is shown that the electric field introduced by the positive fixed oxide charge density typically found at thermally oxidized silicon surfaces and the favorable work function difference between the gate metal aluminum and silicon leads to a reduction of Seff to values well below 1 cm/s at AM1.5 illumination for n-type as well as p-type silicon. At low illumination levels, however, oxidized n-type silicon has much better surface passivation properties than p-type silicon due to the small hole capture cross section (σn/gsp ≈ 1000 at midgap). Only at small illumination intensities for p-type substrates or in the case of poor SiSiO2 interface quality the incorporation of a gate electrode on the rear surface is a promising tool for further reducing surface recombination losses.

Admittance spectroscopy of boron doped diamond

The boron acceptor level in diamond was investigated using admittance spectroscopy. The conductance of a flame grown sample was measured between 125 and 200 K at five frequencies between 0.1 and 5.0 kHz using a 16.0 mV ac signal applied across a Schottky diode at zero dc bias. The admittance spectroscopy technique yielded a deep impurity level of 0.33 eV. From the same set of data, a hole capture cross section of 2×10−12 cm2 was also measured. The cross section reported here is assumed to be caused by ionized boron acceptors.

High-Purity Germanium Technology for Gamma-Ray and X-Ray Spectroscopy

ABSTRACTHigh-purity germanium (HPGe) for gamma-ray spectroscopy is a mature technology that continues to evolve. Detector size is continually increasing, allowing efficient detection of higher energy gamma rays and improving the count rate and minimum detectable activity for lower energy gamma rays. For low-energy X rays, entrance window thicknesses have been reduced to where they are comparable to those in Si(Li) detectors. While some limits to HPGe technology are set by intrinsic properties, the frontiers have historically been determined by the level of control over extrinsic properties. The point defects responsible for hole trapping are considered in terms of the “standard level” model for hole capture. This model originates in the observation that the magnitude and temperature dependence of the cross section for hole capture at many acceptors in germanium is exactly that obtained if all incident s-wave holes were captured. That is, the capture rate is apparently limited by the arrival rate of holes that can make an angular-momentum-conserving transition to a s ground state. This model can also be generalized to other materials, where it may serve as an upper limit for direct capture into the ground state for either electrons or holes. The capture cross section for standard levels σS.L. is given bywhere g is the degeneracy of the ground state of the center after capture, divided by the degeneracy before capture. Mc is the number of equivalent extrema in the band structure for the carrier being captured, mo is the electronic mass, m* is the effective mass, and T is the temperature in degrees Kelvin.

Effect of Doping on the Interface States in Au Schottky Contact to p-In0.21Ga0.79As Grown on GaAs by Metal Organic Vapour Phase Epitaxy

ABSTRACTEpitaxial p-In0.21Gao0.79As/Au Schottky barrier type diodes were fabricated by evaporation of Au on chemically etched surfaces of In0.21Ga0.79As:Zn layers grown on highly doped GaAs substrate by MOVPE. 1 MHz capacitance-voltage (C-V) and Capacitance-frequency (C-f) measurements were performed in the frequency range 1 KHz-1 MHz at room temperature in Au Schottky diodes made on four epitaxial In0.2 Ga0.79As:Zn samples with doping concentrations between 6×1014cm−3 and 4×1017 cm−3. Under forward bias, a large frequency dispersion in the junction capacitance was observed which was attributed to the interface states in thermal equilibrium with the semiconductor. The interface states capacitance extracted from the C-f data was analyzed in terms of Lehovec's theoretical model of interface state continuum with single time constant, and the characteristic parameters of the interface states (energy density (Nss), relaxation time (τ) and hole capture cross-section (σh)) were determined. In the samples with doping concentration in the range 1.5×1017-4.3×1017 cm−3, Nss was about an order of magnitude higher than in the sample having a doping concentration of 5.8×1014 cm−3. Over the interface states energy range 0.40-0.65 eV, Nss decreased exponentially with energy in the highly doped samples and σh increased with energy in all the samples.

Minority carrier capture cross section of the EL2 defect in GaAs

The EL2 defect in GaAs possesses two levels E1 and E2 located in the gap at 0.75 eV below the conduction band (E1) and 0.5 eV above the valence band (E2) for which only the majority carrier (electron for E1 and hole for E2) capture cross sections have been determined. We present a study in which the occupancy of the E1 level under minority carrier injection, monitored by deep level transient spectroscopy, provides the value of the capture cross section of holes on the E1 level. This enables us to estimate the contribution of the EL2 defect to the minority carrier lifetime in GaAs materials

Isothermal capacitance transient spectroscopy measurements on polycrystalline diamond/hydrogenated amorphous silicon heterojunctions

A deep level in boron-doped polycrystalline diamond films located approximately 0.6 eV above the valence-band edge has been found using isothermal capacitance transient spectroscopy (ICTS) measurements. p-n heterojunctions between polycrystalline diamond and hydrogenated amorphous silicon were used in the study. The density and the hole-capture cross section of the deep level traps were determined from the temperature dependence of ICTS spectra and found to be 2×1016 cm−3 and 1×10−17 cm2, respectively.

Hole capture at the DX(Si) and DX(Te) defects in AlxGa1−xAs

Results of measurements of the hole recombination process at DX centers related to group-IV (silicon) and group-VI (tellurium) donor elements in AlxGa1−xAs are presented. For the DX(Si) and DX(Te) defects the values of hole capture cross section σh=2×10−13 and σh=10−13 cm2 were found, respectively. In both cases no systematic change of σh with temperature was observed. The results show that the DX center in the ground state is negatively charged as a consequence of its negative-U two-electron character.

A charge pumping method for rapid determination of interface-trap parameters in metal-oxide-semiconductor devices

We present a new implementation of charge pumping for rapid characterization of interface-trap parameters, especially geometric mean of electron and hole capture cross sections, in metal-oxide-semiconductor field-effect transistors (MOSFETs). With the help of an HP8116A (or equivalent) function generator operated in the voltage-controlled-oscillator mode, this method enables measurement of the charge pumping current as a function of frequency with one continuous sweep of the frequency. Data analysis can be performed on an HP4145B (or equivalent) parameter analyzer by defining user functions. The measurement setup can be readily assembled with standard instruments without the need of a computer. It allows fast measurements without compromising the accuracy. The improved measurement speed has led to new observations revealing the rapid change of the capture cross sections of the interface traps shortly after Fowler–Nordheim hot-carrier injection.

Numerical simulation of 3-level charge pumping

The 3-level charge-pumping technique for characterizing electron and hole capture cross sections of fast interface traps in metal-oxide−semiconductor devices is studied via quasistatic one-dimensional numerical simulation. In general, these simulations confirm our understanding of the operation of the 3-level technique for carrier capture and emission. The simulations show that the previously developed simplified analysis of 3-level charge pumping is reasonably accurate in the emission regime. In contrast, previous simple analysis of the capture regime is found to be in error. For quantitatively accurate results, the effect of the trapped charge on band bending must be included. A revised analytical approach for the capture regime is developed which corrects this error.

DEEP LEVEL IN BOTH Si/SiO2 INTERFACE AND ITS NEIGH-BOURHOOD AND Si/SiO2 INTERFACE STATES IN p TYPE SILICON MOS STRUCTURE

The deep level in both Si/SiO2 interface and its neighbourhood and Si/SiO2 interface states have been studied systematically with Deep Level Transient Spectroscopy (DLTS). Experimental results show that a dominant deep level, H (0.494), exits in both Si/SiO2 interface and its neighborhood in MOS structure formed by thermal oxidation. The deep level possesses some interesting properties, for example, its DLTS peak height depends strongly on temperature, and when the gate voltage reduces the interval between. Fermi-level and Si valence band to a value less than that between the deep level and Si valence band, its sharp DLTS peak is still observable. The hole capture cross section of the interface states at Si/SiO2 interface has been found to depend on temperature, measured with a new method. The energy distribution of the interface states measured with DLTS contradicts to the distribution obtained by quasistatic C-V technique. A new physical model of Si/SiO2 interface was proposed, with this model the above stated experimental results can be successfully expounded.

Conductance technique measurements of the density of interface states between ZnS:Mn and p-silicon

Capacitance and conductance measurements are presented for dc-driven Au/ZnS:Mn/p-Si electroluminescence metal-insulator-semiconductor (MIS) devices, where the ZnS:Mn films are deposited by radio frequency sputtering. Stable dc operation is achieved by introducing oxygen into the film during deposition and subsequently annealing. The effect of the post-deposition annealing upon the density of states at the ZnS:Mn/p-Si interface is investigated. As deposited, the devices show unusual MIS C-V characteristics, that indicate a very high interface state density. Annealing at 700 °C, normal C-V characteristics are observed, indicating that the very high density of states is greatly reduced. For these films the conductance technique has been used to measure the density of states at the interface between the ZnS:Mn and p-Si. The statistical model is found to describe most accurately the interface state conductance response. The interface state density consists of a tail of states that varies between 3.7×1013 cm−2 eV−1 at the silicon Fermi level and 1.1×1013 cm−2 eV−1 at the silicon mid-gap. A small peak is superimposed upon this tail at (−0.16±0.01) eV below mid-gap. The tail of states is believed to be intrinsic to the ZnS:Mn/p-Si interface, but evidence suggests that the small peak is due to the presence of oxygen, which is shown by secondary-ion mass spectrometry analysis to accumulate at the interface after annealing at 700 °C. It seems likely that the very high density of interface states in as deposited devices is a consequence of a plasma damage to the silicon surface during growth, creating defects such as silicon dangling bonds. One possible explanation for the decrease in this density is that by annealing at 700 °C, oxygen in the bulk of the film diffuses to the interface, where it mops up these defects by forming compounds such as SiOx. A simpler model of interface recrystallization is also suggested. The doping density in the depletion region of the silicon is calculated as (7.5±0.5)×1014 cm−3, and the interface state capture cross section for holes is found to have mean value of approximately 10−15 cm−2.

Analysis of recombination centers in (AlxGa1−x)0.5In0.5P quaternary alloys

We studied nonradiative recombination centers in undoped (AlxGa1−x)0.5In0.5Pgrown by metalorganic vapor phase epitaxy using transient capacitance spectroscopy. We found three deep energy levels, including a mid-gap level. We drew an equation to get a capture cross section for minority carriers, and obtained it using isothermal capacitance transient spectroscopy measurement. The mid-gap level had an electron capture cross section of 2 × 10−10 cm2 and a hole capture cross section of 1 × 10−15 cm2. The time constant of nonradiative recombination through the mid-gap level was found to be comparable to that of radiative recombination. We concluded that the mid-gap level is an effective nonradiative recombination center that reduces photoluminescence intensity.

The effects of nitrogen impurity on the radiation detection properties of synthetic diamond

Abstract This contribution examines the influence of nitrogen impurity on the radiation detecting properties of synthetic diamonds. The relatively low sensitivity of the commonly available synthetic diamonds, when used as dosimeters/detectors, is attributed to the presence of nitrogen. A systematic decrease of the nitrogen level shows an increase in the radiation detection sensitivity of these crystals, and below the 25–50 ppm level the increase is even steeper. Experimental evidence strongly suggests that the nitrogen, which is generally accepted to appear in synthetic diamond in dispersed paramagnetic form, not only acts as a radiationless recombination centre for charge carriers (with an estimated hole capture cross section of ≈10−12 cm2), but also affects the amount of traps formed in the synthesis of diamonds. This dual role of nitrogen which is suggested by the accumulated data is found to be consistent with a two-state model that is composed of a trapping and a recombination centre.

Measurement of mobility-lifetime products in amorphous semiconductors

Charge-carrier drift-mobility-lifetime (μτ) product, the range of the carrier, is one of the most important electronic properties of a semiconductor material for device applications. The determination of μτ is, therefore, of fundamental importance in the characterization of amorphous semiconductors and has been the key issue in a number of recent papers. This paper describes two experimental techniques for μτ product measurements and their application to a-Se and Cl-doped Se:0.35%As electro-radiographic films. Xerographic measurements involve corona charging the surface of an amorphous semiconductor film to a voltage Vo. The film is then exposed to a highly absorbed step illumination at the end of which the residual potential, VR1, on the surface is measured. VR1/Vo is then related to μτ. In the interrupted field time-of-flight (IFTOF) measurement technique, during the flight of the photoinjected holes through the specimen, the applied field is removed at time T1 for an interruption duration of ti and then reapplied and the remaining holes extracted. The fractional recovered photocurrent at the end of the interruption time, ti was found to follow the deep-trapping kinetics described by i(T1 + ti)/i(T1) = exp (−ti/τ) where τ is the charge-carrier lifetime. By interrupting the time-of-flight photocurrent at different locations, the technique allows for the determination of the trapping time τ as a function of location across the specimen and hence the examination of sample inhomogeneities. Xerographic residual potential and IFTOF experiments carried out on a range of a-Se and Cl-doped a-Se:0.35%As electroradiographic films show that there is a good correlation between the trapping time determined from xerographic measurements and that determined from IFTOF. The results are discussed in terms of carrier-trapping processes in amorphous semiconductors. The capture cross section of deep hole traps in a-Se and Cl-doped a-Se:0.35%As is determined from the measurement of the hole-mobility-lifetime product and the saturated cycled-up residual potential. Application of the ballistic and diffusional models of Street (Philos. Mag. B, 49, L15 (1984)) indicate that the hole-capture radius in a-Se is 2–3 Å (1 Å = 10−1 m) and that the chemical modification of a-Se by combinational doping with 0.35%As and 10–20 ppm Cl does not affect the basic capture process. The two experimental techniques described represent the most meaningful methods of charge transport and trapping characterization in amorphous semiconductors.

Thermostimulated impurity conduction in characterization of electrodeposited Cu2O films

Electrodeposited films of cuprous oxide show high resistivities in the range of 109–1012 Ω cm. The p-type conductivity, its temperature dependence, and its thermostimulated characteristics can be explained by assuming that a pair of deep levels (acceptor and donor type) control the electrical properties of these films. A novel thermostimulated conductivity model is introduced to include the effect of impurity conduction. Impurity conduction through the acceptor-type level is the dominant transport mechanism at temperatures below approximately 200 K. The experimental results on thermostimulated conductivity measurements reveal the effect of Poole–Frenkel lowering of the ionization energy of the acceptor-type deep level. For a typical sample the zero-field ionization energy of this level is 0.792 eV. Having a concentration of 5×1013 cm−3 and a hole capture cross section of 3.12×10−9 cm2, this level is compensated with a donor-type level of unknown ionization energy having a concentration of 1.89×1013 cm−3. Impurity conduction in this sample shows an activation energy of 0.03 eV before and 0.08 eV after a sample is illuminated at 77 K. From the measurement of the Poole–Frenkel constant the electron affinity of the film is obtained to be 2.9 eV.

Spectroscopic charge pumping: A new procedure for measuring interface trap distributions on MOS transistors

An approach to the application of the charge pumping technique is proposed as a tool for the measurement of interface trap energy distributions in small area MOS transistors. The new approach is spectroscopic in nature, i.e., only one energy window is defined, and forced to move through the bandgap by changing the sample temperature. This method has the advantages of addressing a larger part of the bandgap as compared to the classical approach, of reducing the complication in the processing of the data, and of yielding information about the hole and electron capture cross sections separately. Experiments performed on both n-channel and p-channel MOS transistors reveal that, in the temperature (energy) range studied, the interface-trap distribution is slowly varying with energy and that the trap capture cross section is nearly constant over energy and temperature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical capture cross sections of palladium in silicon

Optical electron and hole capture cross sections for the deep levels Ec−0.22 eV and Ev+0.33 eV associated to Pd in Si have been measured using constant-capacitance deep level optical spectroscopy and optical admittance spectroscopy. The experimental results fit well to the model of Lucovsky. The threshold energies obtained for the electron and hole emission from the Ec−0.22 eV level are 250 and 890 meV, respectively. The threshold energies obtained for the electron and hole emission from the Ev+0.33 eV level are 820 and 330 meV, respectively.

Characterization of rapid thermally nitrided SiO2/Si interface by the conductance technique

Device quality SiO2 films with a thickness of 15 nm have been thermally nitrided in NH3 by a rapid thermal processing technique. The properties of the interface between these films and a Si substrate have been investigated by a conductance technique. The results show that the nitridation increases the density and time constant of interface states and enhances the fluctuation of surface potential, but changes the hole capture cross section only slightly. Specifically, nitridation introduces a peak of interface states at 0.25 eV below midgap and the energy dependency of hole capture cross section is suppressed. Using a patchwork model, the surface potential fluctuation can be well simulated and a surface charge nonuniformity with a long-wavelength distribution may exist. These are consistent with the fact that nitridation induces a high oxide charge density. Experimental data show that all these properties depend on nitridation time and temperature.

Determination of interface trap capture cross sections using three-level charge pumping

A modified three-voltage-level charge pumping (CP) technique is described for measuring interface trap parameters in MOSFETs. Charge pumping (CP) is a technique for studying traps at the Si-SiO/sub 2/ interface in MOS transistors. In the CP technique, a pulse is applied to the gate of the MOSFET which alternately fills the traps with electrons and holes, thereby causing a recombination current I/sub cp/ to flow in the substrate. With this technique, interface trap capture cross sections for both electrons and holes may be determined as a function of trap energy in a single device. It is demonstrated that a modified three-level charge pumping method may be used to determine not only interface trap densities but also to capture cross sections as a function of trap energy. The trap parameters are obtained for both electrons and holes using a single MOSFET.

Electrical and structural study of partially relaxed Ga0.92In0.08As(p+)/ GaAs(n) diodes

The effects of in-plane lattice mismatch have been studied for Ga0.92In0.08As(p+)/GaAs(n)/GaAs(n+) diodes. Different in-plane mismatch at the p–n junction was introduced by a variation of the GaInAs layer thickness (h=0.1, 0.25, 0.5, and 1 μm). Capacitance-voltage (C-V) measurements with different frequencies show a higher-frequency dispersion for a greater lattice-mismatched sample. From the frequency dependence of the C-V curve, single-level charged interface-state density (Ns) was estimated using the effective parallel capacitance and conductance components. The average charged interface density Nss was also estimated using Voltage-intercept (Vint) method. Nss shows a linear dependence on the in-plane mismatch. The charged interface state density is approximately 2.7 Δa∥/a30 for partially lattice-relaxed heterojunctions. For the 1 μm sample, the forward I-V characteristic shows quasi-Fermi level pinning effect. Admittance spectroscopy measurement gives an equilibrium Fermi energy at about Ev+0.36 eV with hole capture cross section cp=2.7×10−15 cm2 for the 1 μm sample and at Ev+0.21 eV and cp=2.4×10−16 cm2 for the 0.5 μm sample.