High Ideality Factor Research Articles

Correlated Nonideal Effects of Dark and Light I–V Characteristics in a-Si/c-Si Heterojunction Solar Cells

a-Si/c-Si (amorphous Silcon/crystalline Silicon) heterojunction solar cells exhibit several distinctive dark and light I-V nonideal features. The dark I-V of these cells exhibits unusually high ideality factors at low forward-bias and the occurrence of a “knee” at medium forward-bias. Nonidealities under illumination, such as the failure of superposition and the occurrence of an “S-type” curve, are also reported in these cells. However, the origin of these nonidealities and how the dark I-V nonidealities manifest themselves under illumination, and vice versa, have not been clearly and consistently explained in the current literature. In this study, a numerical framework is used to interpret the origin of the dark I-V nonidealities, and a novel simulation technique is developed to separate the photo-current and the contact injection current components of the light I-V. Using this technique, the voltage dependence of photo-current is studied to explain the failure of the superposition principle and the origin of the S-type light I-V characteristics. The analysis provides a number of insights into the correlations between the dark I-V and the light I-V. Finally, using the experimental results from this study and from the current literature, it is shown that these nonideal effects indeed affect the dark I-V and the light I-V in a predictable manner.

Luminescence and electrical properties of single ZnO/MgO core/shell nanowires

To neutralise the influence of the surface of ZnO nanowires for photonics and optoelectronic applications, we have covered them with insulating MgO film and individually contacted them for electrical characterisation. We show that such a metal-insulator-semiconductor-type nanodevice exhibits a high diode ideality factor of 3.4 below 1 V. MgO shell passivates ZnO surface states and provides confining barriers to electrons and holes within the ZnO core, favouring excitonic ultraviolet radiative recombination, while suppressing defect-related luminescence in the visible and improving electrical conductivity. The results indicate the potential use of ZnO/MgO nanowires as a convenient building block for nano-optoelectronic devices.

Ni/SiC–6H Schottky Barrier Diode interfacial states characterization related to temperature

This study presents a Ni/SiC–6H Schottky Barrier Diode (SBD) characterization at different temperatures going from 77K to 450K. The electronic properties of this diode were reported by the analysis of its C(VG) and I(VG) characteristics as a function of temperature.At low temperature when T<100K the high part ideality factors nH were close to 2 showing that the conduction is dominated by the generation–recombination at deep centers. Also, the values of low part ideality factor nL varied from 2.69 down to 1.89. These values were also much closer to 2, showing that the conduction mechanism was then dominated by a tunneling current assisted by default.The mean interfacial states density Ds(mean) decreased with increasing temperature from 1.2×1013eV−1cm−2 to 6.3×1012eV−1cm−2. This reducing appeared to be due to the restructuring and rearrangement which occurs under molecules thermal activation within the Ni/SiC–6H metal/semiconductor interface.

Electrical and Photoelectrical Properties of Copper (II) Complex/n-Si/Au Heterojunction Diode

In this study, we fabricated copper(II) complex/n-Si/Au organic-inorganic heterojunction diode by forming copper(II) complex thin film on n-type silicon. A direct optical band gap energy values of the copper(II) complex (Cu2C34H34N2O21Cl4) thin film on a glass substrate was obtained as Eg=2.98 eV. The current-voltage (I-V) measurement of the diode was carried out at room temperature and under dark. The ideality factor n and barrier height ϕb values of the diode were found to be 3.17 and 0.71 eV, respectively. The diode indicates non-ideal current-voltage characteristics due to the high ideality factor greater than unity. The series resistance Rs and ideality factor n values were determined using Cheung’s method and obtained as 5.54 kΩ and 3.81, respectively. The capacitance-voltage (C-V) measurements of the diode were performed at different frequency and room temperature. From the analysis of the C-V measurements carrier concentration Nd, diffusion potential Vd and barrier height values ϕbc-v were determined as 2.79x1015 cm-3, 1.078 V, 1.31 eV, respectively. From the I-V measurements of the diode under 1.5 AM illumination, short circuit current (Isc) and open circuit voltage (Voc) have been extracted as 12.8 µA and 153 mV, respectively.

Rectification properties of nanocrystalline diamond/silicon p-n heterojunction diodes

Carrier transport mechanism in n-type nanocrystalline diamond (NCD)/p-type Si heterojunction diodes prepared by microwave plasma-enhanced chemical vapor deposition is studied in a temperature range of room temperature to 473 K. Current-voltage measurements show at most three orders of magnitude of rectification at ±20 V of biasing and room temperature, depending upon the deposition temperature. The current-voltage characteristics are described with the high ideality factor and the low current injection barrier due to the disordered NCD/Si heterojunction interface, mainly associated with grain boundaries in the NCD film. The Arrhenius plots of the currents reveal that the thermal excitation of carriers limits the conduction, and the apparent activation energy decreases drastically upon the bias voltage change from reverse to forward. The current injection mechanism at the interface is explained along the predicted energy-band diagrams, such that the major carriers from the defect states of the NCD are injected into the conduction band of the Si by forward biasing.

Fabrication of p-type ZnO nanorods/n-GaN film heterojunction ultraviolet light-emitting diodes by aqueous solution method

Zinc oxide (ZnO) is a wide-bandgap material with excellent optical properties for optoelectronics applications. ZnO nanostructures are attractive for research because it is easy to fabricate in single-crystalline form and it has interesting physical properties at the nanoscale. In this paper, we report our successful growth of a p-type ZnO nanorods/n-GaN film heterojunction ultraviolet light-emitting diode (LED). The heterojunction LED shows its advantages over a p-ZnO film/n-GaN film heterojunction. The LED demonstrates a rectifying I–V characteristics with a turn-on voltage of 2.7 V. The ideality factor is 6.5. The existences of interface charges in the interface are the reason for this low turn-on voltage and high ideality factor in the heterojunction. Electroluminescence (EL) spectra of the LED consist of an ultraviolet peak at 378 nm and a broad yellow emission centered at 560 nm. Fitting and comparing EL of the LED with PL of p-ZnO and n-GaN show that p-ZnO contributes more to the EL than n-GaN.

EXPERIMENTAL CHARACTERIZATION AND MODELING ANALYSIS ON NPN AlGaN/GaN HBT WITH HIGH IDEALITY FACTOR IN BOTH COLLECTOR AND BASE CURRENT

Characterization and modeling analysis on both ideality factor of the collector current (η C ) and the base current (η B ) have higher than the excepted values of 1.0 and 2.0, respectively, for npn Al GaN/GaN heterojunction bipolar transistors (HBTs) have been reported. We employ the rapid thermal process annealing (RTP-annealing) to modify the base parasitical Schottky diode (called A-HBTs) after the as-deposited Ni/Au bilayers on the base layer for electrode with no annealing (called N-HBTs) to compare with each other. For a HBT operated in Gummel-plot configuration, experimental and modeling results indicate that the base parasitical Schottky diode (BPSD) causes the base current (I B ) and collector current (I C ) with high ideality factor and raise the base-emitter voltage (V BE ) to higher operation point, and therefore lead to more power consumption. Furthermore, the extended Ebers–Moll equivalent-circuit model together with the extracted device parameters provided simulated results that were in a good agreement with experimental ones.

Local micro-photoreflectance spectroscopy measurements on type II InGaAlAs/GaAsSb/InP heterojunction bipolar transistor: Correlation with electrical characteristics

Micro-photoreflectance (micro-PR) is performed on patterned type II InGaAlAs/GaAsSb/InP heterojunction bipolar transistor to locally measure the built-in electric fields. The results show that the efficiency of the electric field modulation correlates with the ideality factor extracted from the electrical characteristics (Gummel characteristics). The Franz-Keldysh oscillations (FKO) completely disappear at the emitter/base heterojunction on devices with high ideality factor (nearly 2), whereas typical FKO spectra are seen on samples with ideality factor ∼1. Such behavior is attributed to the type II recombination across the InGaAlAs/GaAsSb interface which can reduce the photovoltage effect. Prior to the micro-PR experiments, photoluminescence is performed to demonstrate that the InGaAlAs/GaAsSb interface nature is of type II as well as to estimate the band offset discontinuity ΔEC.

Interface state density distribution in Au/n-ZnO nanorods Schottky diodes

Interface states density (NSS) distribution is extracted in Au/ ZnO Schottky diodes. Nanorods of ZnO are grown on silver (Ag) using aqueous chemical growth (ACG) technique. Well aligned hexagonal–shaped vertical nanorods of a mean diameter of 300 - 450 nm and 1.3 -1.9 μm high are revealed in SEM. Gold (Au) Schottky contacts of thickness 60 nm and 1.5mm diameter were evaporated. For electrical characterization of Schottky diodes current-voltage (I-V) and capacitance-Voltage (C-V) measurements are performed. The diodes exhibited a typical non-linear rectifying behavior with a barrier height of 0.62eV and ideality factor of 4.3. Possible reasons for low barrier height and high ideality factor have been addressed. Series resistance (RS) has been calculated from forward I-V characteristics using Chueng's function. The density of interfacial states (NSS) below the conduction band (EC-ESS) is extracted using I-V and C-V measured values. A decrease in interface states density (NSS) is observed from 3.74 × 1011 − 7.98 × 1010 eV−1 cm−2 from 0.30eV - 0.61eV below the conduction band edge.

A collector current model for InAlAs/InGaAsSb/InGaAs double heterojunction bipolar transistors with non-ideal effects

A collector current model incorporating electron diffusion in the base and thermionic emission at the abrupt B–E heterojunction is derived and applied to InGaAsSb heterojunction bipolar transistors (HBTs). Parameters extracted from the model include effective base doping concentration, conduction band discontinuity (ΔEC) at the base-emitter junction, and emitter resistance. The calculated current from the model is consistent with the measured result. It is found that a high collector current ideality factor is resulted from a nonzero ΔEC and a low effective base doping concentration. Moreover, a nonzero ΔEC makes the high-injection effect almost invisible in collector current characteristics, even if it actually exists.

Electrical Characteristics of Pt Schottky Contacts Fabricated on Amorphous Gallium Indium Zinc Oxides

The electrical characteristics of Pt Schottky diodes fabricated on amorphous gallium indium zinc oxide were investigated. On the basis of Schottky theory with the thermionic emission mode, an effective Schottky barrier height (SBH) of 0.55 eV and an ideality factor of 3.38 were obtained. The anomalously high ideality factor could be attributed to the statistical potential variations of conduction band edges, as evidenced from the distinctive carrier transport through percolation hopping conduction. In this respect, the barrier inhomogeneity model was applied to obtain reasonable Schottky parameters, yielding the mean barrier height of 1.23 eV with a large standard deviation of 192 mV.

Gaussian distribution of Schottky barrier heights on SnO2 nanowires

ABSTRACTIn this work, we studied metal/SnO2 junctions using transport properties. Parameters such as barrier height, ideality factor and series resistance were estimated at different temperatures. Schottky barrier height showed a small deviation of the theoretical value mainly because the barrier was considered fixed as described by ideal thermionic emission-diffusion model. These deviations have been explained by assuming the presence of barrier height inhomogeneities. Such assumption can also explain the high ideality factor as well as the Schottky barrier height and ideality factor dependence on temperature.

Impact of surface topography and laser pulse duration for laser ablation of solar cell front side passivating SiNx layers

Local contact openings in SiNx layers that passivate the front side of solar cells offer an attractive alternative to the current standard “fire-through” screen printing process for front grid fabrication. Additionally, this technology can be used for enabling a selective emitter. In the present paper, we investigate laser ablation of SiNx layers on planar and textured silicon surfaces for various laser wavelengths and pulse durations in the nanosecond (ns) to femtosecond (fs) range. We characterize the dark J-V characteristics of diodes with laser contact openings in the SiNx layer passivating the emitter. Our results show that on alkaline textured surfaces the ablation by a ns laser produces less damage than by an ultrashort pulse laser. The dark currents of alkaline textured diodes treated with picosecond (ps) or fs lasers are one order of magnitude higher than those of ns laser treated diodes. High ideality factors furthermore indicate crystal damage in the ∼500 nm deep space charge region of the diodes. Scanning electron microscope and transmission electron microscope images of textured samples, confirm the presence of extensive and deep crystal damage after ps laser ablation, which are not observed in laser treated samples with planar surfaces. Correspondingly, for planar surfaces we find for both, ns and for ps laser ablated regions, emitter saturation current densities J0e,abl of ∼2 pA/cm2. The recombination in textured samples in contrast differs vastly for ns and ps laser ablation. The ns laser results in an only slightly increased value of 3.7 pA/cm2 while the ps laser treated sample was not evaluable due to severe crystal damage leading to effective lifetimes of &amp;lt;5 μs.

Rectifying and Photovoltage Properties of ZnO:A1/p-Si Heterojunction

An A1-doped ZnO/p-Si heterojunction is fabricated by a laser molecular beam epitaxy technique. The abnormally high ideality factors (n ≫ 2) of the prepared heterojunction are observed in the interim bias voltage range. A theoretical model is proposed to understand the much higher ideality factor of the special heterojunction diode. The ZnO:A1 film shows metal-like conductivity with the electrical resistivity about 6.56 × 10−4 Ω·cm at room temperature. The temperature dependence of the photovoltage indicates that the photovoltaic effect of the A1-doped ZnO based heterojunction can be changed by the intrinsic metal-semiconductor transition at 120 K.

Interface state density of free-standing GaN Schottky diodes

Schottky diodes were fabricated on the HVPE-grown, free-standing gallium nitride (GaN) layers of n- and p-types. Both contacts (ohmic and Schottky) were deposited on the top surface using Al/Ti and Pd/Ti/Au, respectively. The Schottky diode fabricated on n-GaN exhibited double barriers with values of 0.9 and 0.6 eV and better performance in the rectification factor together with reverse and forward currents with an ideality factor of 1.8. The barrier height for the p-GaN Schottky diode is 0.6 eV with an ideality factor of 4.16. From the capacitance–voltage (C–V) measurement, the net doping concentration of n-GaN is 4 × 1017 cm−3, resulting in a lower reverse breakdown of around −12 V. The interface state density (NSS) as a function of EC−ESS is found to be in the range 4.23 × 1012–3.87 × 1011 eV−1 cm−2 (below the conduction band) from Ec-0.90 to EC-0.99. Possible reasons responsible for the low barrier height and high ideality factor have been addressed.

The origin of the high ideality factor in AlGaN-based quantum well ultraviolet light emitting diodes

The ideality factor of AlGaN-based quantum well ultraviolet light emitting diodes (LEDs) is found to be dependent on both material quality and the presence of electron blocking layer (EBL). The ideality factor of the 340 nm LEDs decreases from 6.9 to 4.9 in the low bias regime (1 ≤ V < 2) as the structural dislocation density reduces from 5 × 10 9 to 9 × 10 8 cm -2 . Moreover, the ideality factor of the 310 nm LEDs decreases with increasing thickness of the AlGaN EBL which is placed between the barrier after the QW and the p-type layer. The slope of the I-V characteristics is temperature independent, indicating that the carrier tunneling is the dominant mechanism. The characteristic tunneling energy extracted from the I-V characteristics decreases from the order of 200 to around 100 meV as the dislocation density in the LED is reduced and with the insertion of a 10 nm EBL. This is attributed to the suppression of deep level states assisted electron tunneling into p-type layer.

I– V characteristics of a-Si–c-Si hetero-junction diodes made by hot wire CVD

p-Type hydrogenated amorphous silicon (a-Si:H) was deposited on n-type crystalline silicon (c-Si) substrates to obtain hetero-junction diodes. Additionally, a thin intrinsic a-Si:H layer was inserted between both the p-type film and the n-type substrate to study its passivation effect on the c-Si surface. The amorphous films were obtained by the hot wire chemical vapor deposition (HWCVD) technique, using a tungsten filament and silane (SiH 4), hydrogen (H 2) and diborane (B 2H 6) gases, where the deposition parameters such as gas flow, substrate temperature and filament temperature were varied. Optical band gap, deposition rate and conductivity were measured for all the films. We studied the influence of the quality of the amorphous films upon the performance of the hetero-junction diodes. In particular, the diode ideality factor ( n) and the saturation current density ( J 0) were determined by measuring the current–voltage characteristics in dark conditions. It is shown that the presence of the intrinsic layer is fundamental for making good diodes, since devices made without this film cause the diodes to have high saturation current density and ideality factor ( J 0>10×10 −6 A/cm 2, n>4) as compared to diodes with a good intrinsic layer ( J 0=5×10 −9 A/cm 2, n=1.39). The results obtained are encouraging, but the quality of the intrinsic films still should be improved for applying them to HIT solar cells.

Temperature dependence of current–voltage characteristics of gold–strontium titanate thin film Schottky diode

Gold–strontium titanate (Au–STO) Schottky diode is fabricated using pulsed laser deposition technique. The current–voltage characteristics of the device show non-linear behavior. The junction parameters such as ideality factor, barrier height, and series resistance are calculated using Cheung and Norde's methods. The effect of temperature on diode parameters is studied in details. It is observed that the barrier height varies almost linearly with temperature and the values increase from 0.39 to 0.72 eV in temperature range 150–300 K. The ideality factor decreases from 18 to 3 with increase in temperature from 150 to 300 K. High ideality factor suggests formation of non-ideal Schottky junction between gold and STO.

1.3 μm emitting GaInNAs/GaAs quantum well resonant cavity LEDs

The electrical and optical characteristics of a set of MBE-grown 1.3μm emitting GaInNAs/GaAs quantum well (QW) resonant cavity light-emitting diodes (RCLEDs) have been analyzed as a function of the growth temperature of the distributed Bragg reflector (DBR) and the contact alloying. The RCLEDs consist of a λ-long microcavity delimited on one side by an 8-pair AlAs/GaAs DBR and on the opposite side by a metallic mirror, with the QW located at one of the antinodes of the standing wave. The structures are designed for extraction of the light through the substrate. Room-temperature electroluminescence at 1.3μm is demonstrated with 38% In and 1.3% N in the quantum well. It is found that the RCLEDs with the DBR grown at the lowest temperature of 670°C show a lower differential slope efficiency and degraded current–voltage characteristics, i.e. higher ideality factor, lower shunt resistance and lower threshold voltage. This degradation can be explained to arise from the higher roughness observed in the DBRs grown at 670°C, which likely enhance the formation of point defects and compositional inhomogeneities in the quantum well. Alloying of the ohmic contacts also results in a widening of the electroluminescence emission, probably due to the degradation of the top metallic mirror sharpness caused by the diffusion of the metal into the GaAs. The best results in terms of differential slope efficiency and electrical characteristics are obtained when the DBRs are grown at 750°C and the contacts are left non-alloyed.

Temperature dependences of InxGa1−xN multiple quantum well solar cells

In this work, high open circuit voltages (Voc) of In0.2Ga0.8N and In0.28Ga0.72N multiple quantum well solar cells (MQWSCs) are experimentally obtained (2.2 V and 1.8 V, respectively). The Voc of In0.28Ga0.72N MQWSCs is lower than the expected value due to serious indium segregation problems causing more defects in In0.28Ga0.72N films, which is consistent with the observation of a high ideality factor in dark current measurement. The temperature dependence of the Voc and the short circuit current (Jsc) in In0.2Ga0.8N MQWSCs is found to be larger than the corresponding values in In0.28Ga0.72N MQWSCs. It is also observed that higher quantum well energy barrier exhibits a low fill factor of 0.52 due possibly to the loss of electric field and the higher energy barrier. This obtained efficiency increases with temperatures up to 100 °C and then decreases due to competing results between the reduction in Voc and an increase in Jsc.