Generation-recombination Current Research Articles

Degradation behaviors of GaN light-emitting diodes under high-temperature and high-current stressing

A number of commercially available multiple-quantum well (MQW) InGaN/GaN blue LEDs with wavelengths of about 460nm and a power of 1 mW were stressed at temperatures ranging from 25 to 120°C at several accelerated DC currents. Both the forward and reverse current voltage characteristics as well as the electroluminescent spectra of the LEDs were monitored. These effects also resulted in the pronounced degradation of light efficiency and device operation lifetime. We found that the degradation of photonic characteristics, correlated very well with the generation-recombination current which is governed by the defect density. The device degradation is much faster at high temperatures. At nominal operation current and at room temperature, the light intensity degradation reaches a saturation level before the light dyes out. These results shed new lights upon the design and lifetime specifications for the emerging commercial solid-state lighting devices.

Extraction of Subgap Donor States in a-IGZO TFTs by Generation–Recombination Current Spectroscopy

A physics-based generation-recombination current (JG r) spectroscopy is proposed for the extraction of the sub gap donorlike density of states (DOS) of amorphous InGaZnO thin-film transistors. Physics-based Shockley-Read-Hall recombination through the subgap DOS over the bandgap is fully considered, and the potential for the carrier concentration is calculated through the DeAOTS model. The extracted parameters for the exponential deep donorlike states are N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DD</sub> = 5.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> [cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> · eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ] and kT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DD</sub> = 0.115 [eV].

Thermionic Field Emission Explanation for Nonlinear Richardson Plots

Accurate extraction of the Schottky barrier height (SBH) is imperative to the development of low-resistance contacts. Nonlinearity in the Richardson plot generates errors in the extraction of the SBH. Without invoking generation-recombination currents, defect-assisted tunneling, or even inhomogeneity, nonlinearity of the Richardson plot can be explained by thermionic field emission (TFE) and can be evidenced even in lightly doped diodes. An analytical model is proposed whereby the extracted barrier height is actually related to the peak energy of the energy distribution of the emitted electrons and can be extracted from a typical Richardson plot using polynomial regression. A comparison of the proposed model with the conventional model and a popular inhomogeneous model that uses a Gaussian distribution of barrier heights concludes that the proposed TFE model extends the doping range of an accurate extraction of the SBH.

Determining certain physical settings of photocells

This work considers certain incompletely solved problems regarding the determination of parameters of photocells. In particular, the physical reasons for the nonideality of the n-p transition and possible back-point regeneration are analyzed and generation-recombination currents formed in the area of bulk transition charge are registered. The capabilities of the diffuse (fuzzy) impurity level, nonohmic fall in base voltage, recombination laws in which intercomplex electronic exchange leads to recombination-rate saturation and inhibition (decrease), and development of self-structuring processes in the photocell material are studied as well.

Proton Irradiation of 4H-SiC Ultraviolet Single Photon Avalanche Diodes

The effects of proton irradiation on uv 4H-SiC single photon avalanche photodiodes (SPADs) are reported. The SPADs, grown by chemical vapor deposition, were designed for uv operation with dark count rates (DCR) of about 30 kHz and single photon detection efficiency (SPDE) of 4.89%. The SPADs were irradiated with 2 MeV protons to a fluence of 1012 cm-2. After irradiation, the I-V characteristics show forward voltage (&lt;1.9 V) generation-recombination currents 2 to 3 times higher than before irradiation. Single photon counting measurements imply generation-recombination centers created in the band gap after irradiation. For threshold voltage ranging from 23 to 26 mV, the 4H-SiC SPAD showed low DCR (&lt;54 kHz) and high SPDE (&gt;1%) after irradiation. The SPADs demonstrated proton radiation tolerance for geosynchronous space applications.

Transport properties and barrier height evaluation in Ni/InAlN/GaN Schottky diodes

The current-voltage characteristics of the Ni/InAlN/GaN Schottky diodes were measured at various temperatures in the range of 300–820 K. The experimental data were analyzed considering different current-transport mechanisms, such as thermionic emission, generation-recombination, tunneling and leakage currents. From the fitting of experimental data it follows that the tunneling current dominates in whole temperature range. The thermionic emission becomes comparable to the tunneling current only at highest temperatures used. The barrier height of the Ni/InAlN/GaN Schottky diodes, evaluated from the thermionic emission current, shows a slightly negative temperature coefficient and its value at 300 K is 1.47 eV. Lower barrier heights were reported before, which follow from an incorrect evaluation of measured data without separation of individual current components. The dislocation density of about 2×109 cm−2 is obtained assuming dislocation governed tunneling current mechanism.

Dark Current Mechanism in Bulk GaInNAs Lattice Matched to GaAs

The reverse-bias current-voltage characteristics of a series of Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> ln <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> diodes with bandgap of between 0.87 and 1.04 eV are reported. At low bias, diffusion and generation-recombination currents are dominant at high and low temperatures, respectively. At high reverse bias, the dark current is insensitive to changes in temperature, which is indicative of tunneling current mechanisms. We also observe an exponential dependence of the dark current with the electric field in the mid-bias range for all our diodes, which may be explained by the Poole-Frenkel effect.

Temperature-Dependent $I$– $V$ Characteristics of a Vertical $\hbox{In}_{0.53}\hbox{Ga}_{0.47}\hbox{As}$ Tunnel FET

We report on the experimental temperature-dependent characteristics of vertical In0.53Ga0.47As tunnel field-effect transistors (TFETs) at low drain bias to provide key insight into its device operation and design. Leakage floor (IOFF) is determined by the ungated p+-i-n+ reverse bias leakage and is dominated by Shockley-Read-Hall generation-recombination current. The temperature dependence of subthreshold slope arises from tunneling into mid-gap states at the oxide-semiconductor interface, followed by thermal emission into the conduction band. At intermediate gate voltages, pure band-to-band tunneling dominates, while at higher gate voltages, current transport is diffusion limited. The temperature-dependent study of In0.53Ga0.47As TFET highlights the importance of passivating the III-V and dielectric interface.

STUDY ON γ IRRADIATION EFFECTS OF LONGWAVELENGTH HgCdTe PHOTOVOLTAIC DETECTORS WITH DIFFERENT PASSIVATE LAYERS

The γ irradiation effects of long-wavelength HgCdTe photovoltaic detectors with different passivate structures(one structure was a single layer of ZnS and the other was double layers of CdTe/ZnS) were studied.A real-time measurement of current-voltage(I—V) characteristic was performed during the irradiation process.It is found that the detectors exhibit different irradiation effects as the irradiation dosages increase.Numerical fitting of the resistance-voltage(R-V) curves of detectors was made according to the current mechanism of photovoltaic detectors.It is found that the current is mainly trap-assisted-tunneling current(TAT) at larger reverse bias voltages.And it is generation-recombination current(G-R) at smaller and zero reverse bias voltages.After the analysis of R-V curves of detectors before and after irradiation,it is discovered that the double layers of CdTe/ZnS help to reduce the irradiation displacement effect and bring no obvious increase of indirect trap-assisted-tunneling current as the increase of irradiation dosage.It is also found that irradiation ionization effects are closely related to the initial material parameters of the detectors.Besides,it is also found that the detector with a single layer of ZnS has a higher generation-recombination lifetime of minority carriers by fitting method,and its generation-recombination current increases persistently as the increase of irradiation dosage.

Controlling dark current in type-II superlattice photodiodes

Limiting the defect-mediated dark currents in type-II superlattice (T2SL) IR photodiodes remains the key challenge to focal plane arrays (FPAs) based on this material system. In spite of its larger effective mass to suppress tunneling and more than an order of magnitude longer Auger lifetime, the T2SL photodiode performance still lags behind that of the incumbent HgCdTe-based technology. The tunneling and generation–recombination currents can be strongly suppressed by employing a “W” T2SL structure and gradually increasing the energy gap in the depletion region. For maximum quantum efficiency, this graded-gap geometry is combined in a hybrid structure with two-constituent T2SL absorbers that exhibit roughly twice the diffusion length of the “W” structure. Finally, if the etch used to isolate neighboring pixels is stopped just beyond the junction in the graded-gap device, narrow-gap regions are not exposed and the total sidewall area is reduced by a factor of 20. We have combined all of these approaches to produce a 10.5 μm cutoff FPA with diffusion-limited performance and noise-equivalent differential temperature (NEDT) of 35 mK at 70 K.

Correlation of DC and AC electrical properties of Al/p-Si structure by I– V– T and C( G/ ω)– V– T measurements

Current–voltage–temperature and admittance-gate bias–temperature measurements were performed on Al/p-Si structure to investigate transport and storage properties. In forward direction of former measurement, three distinct voltage regions designated different transport mechanisms: recombination, tunneling and space charge limited current. In the reverse direction, current seemed to be roughly proportional to the square root of bias at low region. Activation energy ( E A) as 0.56 eV together with the ideality factor around 1.5 confirmed that generation–recombination current was the actual current transport mechanism. Increase in applied bias resulted in the appearance of other mechanisms: tunneling and space charge limited current, deduced by temperature-independent current flow and power of bias (greater than 2) in both forward and reverse directions, respectively. On the other hand, the distinctive bias voltage regions in current–voltage–temperature measurement corresponded to inverting, depleting and accumulating gate bias regimes in typical M(I)S structure for admittance analysis. Conductance branch of admittance reflected a similar behavior with that of the measurement under both temperature and frequency as parameter. On the other hand, with the aid of capacitance branch of admittance in the light of mentioned regimes and their corresponding energy band diagrams, correlation with ac and dc electrical properties was carried out.

Characterization of Inversion Tunneling Current Saturation Behavior for MOS(p) Capacitors With Ultrathin Oxides and High-$k$ Dielectrics

The inversion current conduction mechanism for MOS(p) capacitors with ultrathin oxides was analyzed from another aspect of bulk traps in this paper. The relationships between deep depletion and generation-recombination current were also studied. It was found that the generation-recombination current due to bulk traps is proportional to the deep-depletion width and dominates the inversion tunneling current. Moreover, it was observed that the inversion tunneling current levels for SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gate dielectrics were different. This discrepancy was explained with their energy band diagrams. Due to the small conduction-band offset of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the gate dielectrics of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> show a worse capability to block the inversion tunneling current in the saturation region than Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> gate dielectrics.

Specific features of light current-voltage characteristics of p-i-n structures based on amorphous silicon in the case of the tunnel-drift mechanism of dark current transport

Current-voltage (I–V) characteristics of p-i-n structures based on amorphous silicon (α-Si:H) with small hole diffusion lengths (shorter than the thickness of the i-layer of a p-i-n structure) have been experimentally studied with and without illumination. It is shown that forward I–V characteristics of structures of this kind can be described by a dependence inherent in diodes, with a diode ideality factor two-three times the maximum value of 2, theoretically predicted for generation-recombination currents in p-n junctions. The dark current is always substantially lower than the photocurrent in a cell biased with a voltage approximately equal to the opencircuit voltage of the photocell. Dark currents cannot contribute to the I–V characteristic under illumination. The photocurrent decreases with increasing photovoltage at a bias lower than the open-circuit voltage because of a decrease in the collection coefficient and the increasingly important role of back diffusion of electrons into the p-contact, rather than as a result of the dark injection. In the case of biases exceeding the open-circuit voltage, back diffusion becomes the predominant component of the current.

Improved free-standing GaN Schottky diode characteristics using chemical mechanical polishing

Chemical mechanical polishing of free-standing GaN substrates is found to reduce the reverse leakage current of Ni/Au Schottky diodes fabricated on the Ga-face. The barrier height extracted from current–voltage measurements is found to increase from ∼0.4 eV on un-treated substrates to 0.75 eV on polished wafers, along with a reduction in diode ideality factor from 2 to 1.5. The electrical measurements are consistent with removal of a defective near-surface region that promotes generation-recombination current. More acidic polishing solutions were found to produce the best Schottky diode characteristics and there was an optimum loading force during CMP of the GaN surface.

Leakage mechanisms and potential performance of molecular-beam epitaxially grown GaInAsSb 2.4 μm photodiode detectors

We report on the growth, processing, and characterization of uncoated, unpassivated GaInAsSb detector mesa photodiodes with a cutoff wavelength of 2.4 μm. We find peak room temperature specific detectivity value of 6×1010 cm Hz1/2/W and maximum zero-bias resistivity of 25 Ω cm2. The zero-bias resistivities of mesas with areas ranging from 502 μm2 to 1 mm2 were found to be limited by generation-recombination currents at mesa sidewalls. At low temperatures, devices were limited by Ohmic leakage, which is likely due to the formation of oxides on the sidewalls. After 6 months of aging, Ohmic leakage becomes the limiting factor at room temperature as well. Based on experimentally obtained material parameters in this and other studies, an upper limit D∗ and R0A were estimated for GaInAsSb materials and compared to those HgCdTe and InGaAs.

FinFET reliability study by forward gated-diode generation–recombination current

Reliability of FinFETs is studied in this paper using the forward gated-diode generation–recombination (G–R) current. The current–voltage characteristics of a FinFET are measured for parameter extraction and a mathematical algorithm is used to extract the stress-induced interface states and oxide traps of the FinFET from the G–R current measurement. It is observed that the stress-induced interface states and oxide traps can be distinguished by observing the shift of the peak G–R current in the body current (Ib) versus gate voltage (Vg) characteristic. The interface states cause a change in the maximum G–R current (ΔIpeak) while the oxide traps result in a shift of the gate voltage (ΔVg) corresponding to the ΔIpeak. Unlike bulk MOSFETs, the dominant degradation mechanism of FinFETs is found to be the oxide traps formation rather than the interface states generation.

Real-time study of γ irradiation on Hg1-xCdxTe focal plane photodiodes

Real-time measurement of γ irradiation effect of Hg1-xCdx Te long-and mid-wavelength focal plane array photodiodes has been carried out. Through measuring the current-voltage characteristic during irradiation process, it has been found that mid-wavelength detectors are more radiation resistant than long-wavelength photodiodes. For long-wavelength detectors, the zero bias resistance, which is usually used to evaluate the performance of photodiodes, decreases with increased γ irradiation dosage. For mid-wavelength detectors, the zero biased resistance does not show a definite changing trend, and the irradiation mainly caused fluctuations of resistance-voltage curves with increased dosage. By numerically simulating the resistance-voltage curves of long-wavelength detectors on the basis of dark current mechanism, it was found that the lifetime ofminority carriers in the generation-recombination process was shortened and thedefects produced by irradiation increased as the dosage increased, and the affected dark current mechanism was mainly the generation-recombination current. Because the irradiated mid-wavelength detectors have much larger carrier mobility and much lower dopant density, and also a bandgap twice that of long-wavelength detectors, they showed a weaker irradiation effect. The fluctuations of the resistance-voltage curves caused by irradiation would lead to an increase on noise of the detectors.

Analysis of structure parameters and current conduction mechanisms of AlGaN/GaN Schottky contacts

An investigation on the current conduction mechanisms of AlGaN/GaN Schottky contacts is presented in this paper. The ideality factor and the barrier height of Schottky contact on AlGaN/GaN heterostructure are calculated by I-V measurement in a wide temperature range. By subtraction of generation-recombination, tunneling and leakage currents from the total current, the “pure" Schottky barrier height can be evaluated with higher physical relevance. After analyzing the variational rules of Schottky contact characteristics on AlGaN/GaN heterostructure at 300—550K, it is concluded that surface leakage current plays an insignificant role in producing the gate leakage current of AlGaN/GaN HEMT.

Generation-recombination effects on dark currents in CdTe-passivated midwave infrared HgCdTe photodiodes

The effect of an abrupt CdTe∕HgCdTe passivation heterointerface on generation recombination and dark currents in n-on-p midwave infrared photodiodes with 5.2-μm cut-off wavelength has been investigated. Experimentally, it was observed that the zero-bias-dynamic resistance, R0, at low temperatures scales with the perimeter of the n-on-p junction, rather than with the junction area, suggesting that surface effects are dominant. The diode current–voltage characteristics at low temperatures indicate significant contributions from tunneling effects, which is the dominant leakage current mechanism for reverse bias greater than approximately 30mV. These two observations suggest that the region where the junction terminates at the CdTe∕HgCdTe abrupt interface is responsible for the above effects. A two-dimensional model has been developed to investigate the dark current mechanisms in the vicinity of the junction termination at CdTe∕HgCdTe interface, which also takes into account the effect of dislocations on generation-recombination processes. Calculated profiles of the energy bands and electric field along different cross sections of the photodiode indicate that the electric field achieves a maximum value of the order of mid −105V∕cm in the region where the junction terminates at the CdTe∕HgCdTe interface. The presence of such a high localized electric field in this area decreases the ionization energy of trap levels in the band gap and, hence, increases efficiency of the Shockley–Read–Hall generation-recombination processes. In addition to diffusion, generation recombination, and trap-assisted tunneling mechanisms, the calculations of dynamic resistance include Poole–Frenkel and phonon-trap-assisted tunneling effects. The best fit to the zero bias dynamic resistance versus temperature results has been obtained using an aerial dislocation density in the bulk of the HgCdTe layer equal to 106cm−2. Although the direct band-to-band tunneling and impact ionization processes were also considered, their contributions are shown to be insignificant.

Electrical characterization of Cd∕CdTe Schottky barrier diodes

We have deposited Cd metal contacts on molecular-beam epitaxy CdTe(112)B on Si(112) and have made electronic transport measurements to deduce the properties of the Schottky barrier diode thus formed. We find a room-temperature barrier height ϕB=0.92eV and an ideality factor n≈2. We suggest that the high value of n is due to generation–recombination currents associated with dislocations in the CdTe epilayer.