Temperature-dependent Current-voltage Measurements Research Articles

Investigation of temperature dependent barrier height of Au/ZnO/Si schottky diodes

In this study, temperature dependent current-voltage (I-V) measurements have been performed to investigate the inhomogeneity in the temperature dependent barrier heights of Au/ZnO/Si Schottky barrier diode in the temperature range 150 – 400K. The room temperature values for ideality factor and barrier height were found to be 2.9 and 0.60 eV respectively indicating the inhomogenity in the barrier heights of grown samples. The Richardson plot and ideality factor verses barrier height graph were also drawn to verified the discontinuity between Au and ZnO. This barrier height inhomogenity was explained by applying Gaussian distribution model. The extrapolation of the linear Φap (n) plot to n= 1 has given a homogeneous barrier height of approximately 1.1 eV. Φap versus 1/T plot was drawn to obtain the values of mean barrier height for Au/ZnO/Si Schottky diode (1.1 eV) and standard deviation(δs) (0.02 V) at zero bais.

Forward Current Transport Mechanisms of Ni/Au—InAlN/AlN/GaN Schottky Diodes

We fabricate two Ni/Au-In0.17Al0.83N/AlN/GaN Schottky diodes on substrates of sapphire and Si, respectively, and investigate their forward-bias current transport mechanisms by temperature-dependent current-voltage measurements. In the temperature range of 300–485 K, the Schottky barrier heights (SBHs) calculated by using the conventional thermionic-emission (TE) model are strongly positively dependent on temperature, which is in contrast to the negative-temperature-dependent characteristic of traditional semiconductor Schottky diodes. By fitting the forward-bias I–V characteristics using different current transport models, we find that the tunneling current model can describe generally the I–V behaviors in the entire measured range of temperature. Under the high forward bias, the traditional TE mechanism also gives a good fit to the measured I–V data, and the actual barrier heights calculated according to the fitting TE curve are 1.434 and 1.413 eV at 300K for InAlN/AlN/GaN Schottky diodes on Si and the sapphire substrate, respectively, and the barrier height shows a slightly negative temperature coefficient. In addition, a formula is given to estimate SBHs of Ni/Au—InAlN/AlN/GaN Schottky diodes taking the Fermi-level pinning effect into account.

Investigation of gate leakage mechanism in Al2O3/Al0.55Ga0.45N/GaN metal-oxide-semiconductor high-electron-mobility transistors

The mechanism of both reverse and forward gate leakage currents in Al2O3/Al0.55Ga0.45N/GaN structures was studied in this Letter by temperature-dependent current-voltage measurement. Poole–Frenkel (PF) emission, an oxygen vacancy-assisted process, was deduced as the dominant mechanism at high-temperatures (>388 K), and the leakage current at mid-temperatures (<388 K) were found greatly impacted by temperature-independent tunneling current. The reverse PF mission current in low-field, mid-field, and high-field region were related to trap states with activation energy of 0.41 eV, 0.49 eV, and 0.71 eV, respectively, and the activation energy of trap states for forward PF emission current was derived as 0.65 eV.

High temperature current-voltage characteristics of InP-based tunnel junctions

In this paper, we present temperature-dependent current–voltage measurements of tunnel junctions lattice matched to InP at temperatures ranging from room temperature to 220 °C. Temperature-dependent tunneling properties were extracted by fitting the current–voltage characteristics using a simple analytical formula. Three different designs of tunnel junction were characterized, including a bulk InAlGaAs tunnel junction, an InAlGaAs tunnel junction with InAlAs cladding layers and an InGaAs/InAlGaAs quantum-well tunnel junction. Each device exhibited different temperature dependence in peak tunnel current and excess current, with the quantum-well tunnel junction exhibiting the greatest temperature sensitivity. We use a non-local tunneling model, in conjunction with a numerical drift-diffusion solver, to explain the performance improvement available by using double heterostructure cladding layers around the junction region, and use the same model to explain the observed temperature dependence of the devices. Copyright © 2014 John Wiley & Sons, Ltd.

Defect-related electronic metastabilities in chalcopyrite compounds

So far, in Cu(In,Ga)Se2 solar cells the metastable behavior of the key parameters, i.e. open circuit voltage, short circuit current and fill factor, and the corresponding defect physics were typically investigated independently. In order to contribute to this issue, we systematically varied between annealed and light soaked state and investigated the influence of these processes on the solar cell parameters as well as on the defect physics. In this work, we attempt to correlate the key parameters of the solar cells and the defect physics by discussing experimental results obtained from temperature dependent current–voltage measurements (IVT) as well as from capacitance voltage (CV), admittance (AS) and deep-level transient spectroscopy (DLTS). A commonly observed defect contribution in Cu(In,Ga)Se2 solar cells is the so-called N1 signature. The activation energy of this signature was found to increase upon air-annealing in the dark which goes along with a decrease in the open circuit voltage and the effective doping density. In this paper we will discuss the correlation between annealing-induced shifting of defect energies and the variation of the key parameters.

Subband structure of two-dimensional electron gases in SrTiO3

Tunneling between two parallel, two-dimensional electron gases (2DEGs) in a complex oxide heterostructure containing a large, mobile electron density of ∼3 × 1014 cm−2 is used to probe the subband structure of the 2DEGs. Temperature-dependent current-voltage measurements are performed on SrTiO3/GdTiO3/SrTiO3 junctions, where GdTiO3 serves as the tunnel barrier, and each interface contains a high-density 2DEG. Resonant tunneling features in the conductance and its derivative occur when subbands on either side of the barrier align in energy as the applied bias is changed, and are used to analyze subband energy spacings in the two 2DEGs. We show that the results agree substantially with recent theoretical predictions for such interfaces.

Forward current transport mechanisms in Ni/Au-AlGaN/GaN Schottky diodes

The forward current transport mechanisms in Ni/Au-AlGaN/GaN Schottky diodes are studied by temperature dependent current-voltage (T-I-V) measurements from 298 to 473 K. The zero-bias barrier height qϕBn and ideality factor values determined based on the conventional thermionic-emission (TE) model are strong functions of temperature, which cannot be explained by the standard TE theory. Various transport models are considered to analyze the experimental I-V data. The fitting results indicate that the increased current at low bias is due to the trap-assisted tunneling with an effective trap density of about 8.8 × 106 cm−2, while the high-bias current flow is dominated by the TE transport mechanism, accompanied by a significant series resistance effect. By fitting the high-forward-bias I-V characteristics, the effective qϕBn values with a small negative temperature coefficient are obtained. The temperature dependence of the saturation tunneling current and qϕBn is finally explained by considering the thermally induced band gap shrinkage effect.

First quadrant phototransistor behavior in CuInSe2 photovoltaics

Temperature-dependent current voltage measurements on a CuInSe2 (CIS) solar cell are described and analyzed in detail. At relatively low temperatures, the device is shown to exhibit operation as a phototransistor, evidenced by light flux dependent current in the first quadrant of the current/voltage curve prior to the onset of a high differential resistance region of the curve. The phototransistor behavior is not restricted to light with short wavelengths (such as would stimulate the front contact materials—CdS and ZnO), and has a similar sensitivity to light over a wide range of intensities. Other aspects of the device operation are analyzed including observation of a photoconductive shunt and series differential resistance component, neither of which requires short wavelength light to be observable, a non-light-sensitive diode reverse-saturation current, and characterization of the back diode in the device. It is argued that the presence of an effective back diode also requires a bias voltage to develop across that diode under a variety of operating conditions that could reduce the open circuit voltage of the device. The potential for characterization of minority carrier properties in the device absorber layer through further controlled study of phototransistor behavior in the devices is noted.

Carrier transport mechanisms of n-ZnO/ZnMgO/p-GaN heterojunctions revealed by temperature-dependent current–voltage characteristics

The heterojunction diodes with n-ZnO/ZnMgO/p-GaN structure have been deposited on the commercially available p-GaN/sapphire substrates by a simple process of ultrasonic spray pyrolysis. To demonstrate the effect of ZnMgO layer as electron blocking layer, the carrier transport mechanisms of both n-ZnO/p-GaN and n-ZnO/ZnMgO/p-GaN heterojunctions were systemically investigated by temperature-dependent current–voltage measurements. The results revealed that the multistep tunneling is the dominant carrier transport mechanism in n-ZnO/ZnMgO/p-GaN heterojunctions under forward current, accurately controlled ZnMgO layer thickness can effectively block electron injection from ZnO to GaN and promote hole injection from GaN to ZnO. Therefore, the design and application of ZnO based heterostructure devices will benefit significantly from these achievements.

Electrical characterization of Au/ZnO/Si Schottky contact

In this study, temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements have been performed on Au/ZnO/Si Schottky barrier diode in the range 150 – 400K. The room temperature values for ideality factor and barrier height found to be 2.68 and 0.68 eV respectively. From the temperature dependence of I–V, the ideality factor was observed to decrease with increasing temperature and barrier height increased with increasing temperature. The observed barrier height trend was disagreeing with the negative temperature coefficient for semiconductor. A deep defect with activation energy 0.57 eV below the conduction band was observed using the saturation current plot and deep level transient spectroscopy.

Admittance spectroscopy in kesterite solar cells: Defect signal or circuit response

Unlike Cu(In,Ga)Se2 based solar cells, Cu2ZnSn(S,Se)4 solar cells show a strong increase in series resistance with decreasing temperature. In this study we deduce the series resistance from temperature dependent current-voltage measurements on a 5.5% efficient Cu2ZnSnSe4 solar cell. By applying a simple circuit model an increasing series resistance with decreasing temperature alone results in a capacitance step within the C-f profile. We show that this step needs to be distinguished from a step caused by a defect state or a carrier freeze-out. Consequently, the deduced activation energy is strongly distorted by the circuit response.

Temperature-dependent current–voltage characteristics of Se Schottky contact to n-type Ge

We fabricated Se Schottky contacts to n-type Ge and demonstrated their electrical properties using temperature-dependent current–voltage measurements in the temperature range of 200–300K. As the temperature decreased, the barrier height decreased and the ideality factor increased. These anomalies could be associated with the barrier height inhomogeneities prevailing at the metal–semiconductor contact. On the assumption of the Gaussian distribution of barrier height, the mean value and standard deviation of barrier height, extracted from the plot of barrier height as a function of q/2kT, were found to be 0.70 and 0.089eV, respectively. The electric field dependence of the reverse current revealed that the Schottky emission mechanism dominates the current conduction in the reverse bias.

PtSi dominated Schottky barrier heights of Ni(Pt)Si contacts due to Pt segregation

Temperature dependent current-voltage measurements show that the addition of only 10% Pt to NiSi causes an increase of Schottky barrier height (SBH) from 0.65 eV for NiSi to 0.78 eV for the 10% Pt alloy. Internal photoemission measurements resolve two SBHs in all alloyed samples with ≥5% Pt incorporation corresponding to NiSi and PtSi (∼0.68 eV and ∼0.80 eV), proving that each contributes independently to junction current. High angle annular dark field imaging with scanning transmission electron microscopy confirms Pt segregation to the Ni(Pt)Si/Si interface. The resulting increased SBH may therefore be detrimental to contact resistivity in future technology nodes.

Current-Voltage and Capacitance-Voltage Characteristics of Pd Schottky Diodes Fabricated on ZnO Grown along Zn- and O-Faces

Temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were utilized to understand the transport mechanism of Pd Schottky diodes fabricated on Zn- and O-faces of ZnO. From I-V measurements, in accordance with the thermionic emission mechanism theory, it was found that the series resistance Rsand the ideality factor n were strongly temperature dependent that decreased with increasing temperature for both the faces (Zn and O-face) of ZnO revealing that the thermionic emission is not the dominant process. The barrier height øB(I-V)increased with increasing temperature for both faces. The measured values of ideality factor, barrier height and series resistance for Zn- and O-faces at room temperature were 4.4, 0.60 eV, 217 Ω and 2.8, 0.49 eV, 251 Ω respectively. The capacitance-voltage (C–V) measurements were used to determine the doping concentration Nd, the built-in-potential Vbi, and the barrier height øB(C-V). The doping concentration was found to be decreased with increasing depth. The barrier height øB(C-V)calculated for O-polar and Zn-polar faces decreases with increasing temperature. The values of barrier height øB(C-V)determined from C-V measurements were found higher than the values of barrier height øB(I-V). Keeping in view the calculated values of ideality factor, barrier height, and series resistance shows that O-polar face is qualitatively better than Zn-polar face.

Comparative study of transparent rectifying contacts on semiconducting oxide single crystals and amorphous thin films

We demonstrate fully transparent, highly rectifying contacts (TRC) on amorphous GaInZnO and compare them to TRC fabricated on single crystalline bulk ZnO and heteroepitaxial ZnO thin films. The contacts’ transparency in the visible spectral range exceeds 70%. From numerical simulations, we conclude that thermionic emission is the dominating transport mechanism, however, for several samples with low net doping density diffusion theory must be applied. The detailed investigation of the rectification properties of the TRC using temperature-dependent current-voltage and capacitance-voltage measurements reveals that barrier inhomogeneities govern the IV-characteristics of all diodes irrespective of the sample crystallinity. Assuming a Gaussian barrier height distribution, the extracted mean barrier heights typically range between 1.1 and 1.3 V. The width of the barrier distribution correlates with the mean barrier height and ranges from 110 to 130 mV. By compiling literature data, we found that this correlation holds also true for Schottky diodes on elemental and III-V semiconductors.

Temperature Dependence of Electrical Characterization in n+ - CdS/ p - CdTe Thin Film Solar Cells – Study of Shallow/Deep Defects

ABSTRACTFor CdTe there is no real distinction between defects and impurities exists when non-shallow dopants are used. These dopants act as beneficial impurities or detrimental carrier trapping centers. Unlike Si, the common assumption that the trap energy level Et is around the middle of the band-gap Ei, is not valid for thin film CdTe. Trap energy levels in CdTe band-gap can distributed with wide range of energy levels above EF. To identify the real role of traps and dopants that limit the solar cell efficiency, a series of samples were investigated in thin film n+-CdS/p-CdTe solar cell, made with evaporated Cu as a primary back contact. It is well known that process temperatures and defect distribution are highly related. This work investigates these shallow level impurities by using temperature dependent current-voltage (I-V-T) and temperature dependent capacitance-voltage (C-V-T) measurements. I-V-T and C-V-T measurements indicate that a large concentration of defects is located in the depletion region. It further suggests that while modest amounts of Cu enhance the cell performance by improving the back contact to CdTe, the high temperature (greater than ∼100°C) process condition degrade device quality and reduce the solar cell efficiency. This is possibly because of the well-established Cu diffusion from the back contact into CdTe. Hence, measurements were performed at lower temperatures (T = 150K to 350K). The observed traps are due to the thermal ionization of impurity centers located in the depletion region of p-CdTe/n+-CdS junction. For our n+-CdS/p-CdTe thin film solar cells, hole traps were observed that are verified by both the measurement techniques. These levels are identical to the observed trap levels by other characterization techniques.

Nonvolatile Multilevel Resistive Switching in $ \hbox{Ar}^{+}$ Irradiated $\hbox{BiFeO}_{3}$ Thin Films

Low-energy Ar+ ion irradiation has been applied to an Au/BiFeO3/Pt capacitor structure before deposition of the Au top electrode. The irradiated thin film exhibits multilevel resistive switching (RS) without detrimental resistance degradation, which makes the intermediate resistance states more distinguishable, as compared with the nonirradiated thin film. The stabilization of resistance states after irradiation is discussed based on the analysis of the conduction mechanism during the RS, which was investigated by means of temperature-dependent current-voltage measurement from room temperature to 423 K.

Engineering Electronic Properties of Graphene by Coupling with Si-Rich, Two-Dimensional Islands

Recent theoretical and experimental studies demonstrated that breaking of the sublattice symmetry in graphene produces an energy gap at the former Dirac point. We describe the synthesis of graphene sheets decorated with ultrathin, Si-rich two-dimensional (2D) islands (i.e., Gr:Si sheets), in which the electronic property of graphene is modulated by coupling with the Si-islands. Analyses based on transmission electron microscopy, atomic force microscopy, and electron and optical spectroscopies confirmed that Si-islands with thicknesses of ~2 to 4 nm and a lateral size of several tens of nm were bonded to graphene via van der Waals interactions. Field-effect transistors (FETs) based on Gr:Si sheets exhibited enhanced transconductance and maximum-to-minimum current level compared to bare-graphene FETs, and their magnitudes gradually increased with increasing coverage of Si layers on the graphene. The temperature dependent current-voltage measurements of the Gr:Si sheet showed approximately a 2-fold increase in the resistance by decreasing the temperature from 250 to 10 K, which confirmed the opening of the substantial bandgap (~2.5-3.2 meV) in graphene by coupling with Si islands.

Investigation of Reverse Leakage Characteristics of InGaN/GaN Light-Emitting Diodes on Silicon

We investigate the reverse leakage characteristics of InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown on Si (111) substrate by metal-organic chemical vapor deposition. The reverse leakage characteristics of InGaN/GaN LED on silicon are measured as low as ~10 nA at -5 V and -10 μA at -15 V. Temperature-dependent current-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) measurements of LED devices reveal that the reverse leakage current mechanism is mainly attributed to the field-enhanced thermionic emission, also known as Poole-Frenkel emission, of carriers from deep centers within the space charge region up to ~ -18 V. The analysis of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> curve yields the calculation of the coefficient of the Poole-Frenkel effect (1.12 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> eV·V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> ) and activation energies of carriers (~214 meV at -5 V). With further increase of reverse bias, up to -40 V, LED devices exhibit the onset of space-charge-limited leakage current mechanism without any local breakdown.

An analysis of the effect of illumination to the reverse and forward bias current transport mechanisms in an efficient n-ZnO/n-CdS/p-Cu(In,Ga)Se2 solar cell

Temperature dependent current–voltage measurements are used to determine the effect of white light illumination on the dominant current transport mechanism in an efficient n-ZnO/n-CdS/p-Cu(In,Ga)Se2 heterojunction solar cell. The forward J–V characteristics are modelled by double exponential expression. In the dark, the forward current transport is mainly dominated by recombination at interfacial regions with pronounced tunnelling effect. Nevertheless, approximately above 200K, the thermal dependence of the diode parameters in the second junction interface can be explained in terms of properly increasing contribution of pure depletion region recombination mechanism. Under white light illumination, the forward current is still dominated by the tunnelling enhanced interface recombination mechanism with considerably enhanced tunnelling contribution. The presence of a significant distortion observed below 180K is attributed to the shift of Fermi level at junction interface and recharge distribution in the junction region due to metastable nature of the defect states. Below and above than 230K, the reverse bias current transport in the dark is described by using space charge limited current and a thermally activated behaviour, respectively. Under illumination the variation of temperature dependent reverse current with voltage suggests that either the SCL current in the velocity saturation regime or tunnelling dominates the current transport.