Multi-tunneling Capture-emission Research Articles

Influence of the type of interlayer on current transport mechanisms and defects in n-ZnO/ZnCdO/p-Si and n-ZnCdO/ZnO/p-Si heterojunctions grown by molecular beam epitaxy

A pair of heterojunctions with different type of interlayer: n-ZnO/ZnCdO and n-ZnCdO/ZnO were grown by molecular beam epitaxy on p-Si substrate and investigated in the present work. At first, their electrical properties and parameters were discussed following current-voltage (I-V) characteristics measured over a wide range of temperatures from 40 K to 300 K. The analysis of I-V plots in log-log scale pointed out the presence of different carrier transport mechanisms: tunneling, multi-tunneling capture-emission and space-charge limited currents. Next, the capacitance-voltage characteristics were studied to determine the dopant concentration of the investigated junctions, along with the net doping profiles and their temperature dependence in the range of 40 K to 300 K. Finally, a deep level transient spectroscopy method was applied to characterise defects in the studied junctions. Donor-like traps were identified and their parameters, such as activation energy and capture cross section, were calculated. Furthermore, the influence of cadmium ions in ZnCdO interlayer of n-ZnO/ZnCdO/p-Si diode on the energy position of trap levels was noticed. Moreover, it was demonstrated that the interlayer type affects junction parameters, especially its built-in potential, as well as it modifies the current transport mechanisms in the intermediate range of voltage.

Carrier transport mechanisms in the ZnO based heterojunctions grown by MBE

This paper presents a review of models of the current transport in different kind of heterojunctions (HJs) and their characteristics. In order to effectively deduce the dominant electron transport for the HJs based on ZnO or Zn 1− x Mg x O layers grown on Si substrate by MBE a comparison is performed – which type of the HJ exhibits better electrical properties. The current–voltage characteristics for the studied HJs were measured within 280–300 K. The transport properties of the HJs are explained in terms of Anderson model with reference to aforementioned current transport models. It is found, that the mechanisms of current transport for all of the studied HJs are similar. At a low forward voltage bias the tunneling current dominates while at medium voltage bias (0.5–1 V) multitunneling capture-emission prevails with the electron trap located at 0.1–0.25 eV below the bottom of a ZnO (Zn 1− x Mg x O) conduction band. Beyond this voltage bias space charge limited current governs the current transport.

Fabrication and electrical characterization of thermally deposited amorphous Se0.82In0.18 on n-Si (100)

Amorphous (a-) Se0.82In0.18 thin films have been deposited onto n-type silicon (n-Si) single crystal, using the three-temperature technique, in the fabricated configuration of Au/a-Se0.82In0.18/n-Si/Al. The current density-voltage (J–V) characteristics have been measured at different isotherms in the range of 198–313 K, thus inspecting the conduction mechanisms comparing with Au/a-Se/n-Si/Al heterojunctions. The analysis proved that the forward bias is characterized by two parts: current increasing exponentially with the applied voltage (low voltage bias region, V 0.2 V). At the low bias region, the current was dominated by a multi-tunneling capture-emission process with a rather temperature-independent effect in the temperature range investigated. However, at the high voltage region, the effect of temperature becomes more pronounced with an ohmic character in the range of 198 to 273 K. For temperatures higher than 273 K, and below the glass transition temperature of a-Se0.82In0.18 (T g∼330 K), the high voltage region could be subdivided into two parts: an ohmic conduction range that limited at bias voltage of 0.20 V 0.46 V. The reverse J–V characteristics showed a deviation from that of the ideal diode behavior, analogous to that of pure a-Se/n-Si heterojunctions.

ELECTRICAL PROPERTIES OF P3HT (POLY [3-HEXYLTHIOPHENE])/n-TYPE CRYSTALLINE SILICON (n-c-Si) SOLAR CELLS

Au / P 3 HT (poly [3-hexylthiophene])/n-type crystalline silicon ( n - c - Si ) solar cells have been fabricated. The Aluminum back contact is obtained by evaporation on silicon substrate. An 80 nm P 3 HT layer thick was spin-coated on silicon substrate followed by thermal annealing. Finally golden contacts are deposited by sputtering. The best characteristics of this flawed solar cell are: V oc =0.47 V , I sc =7.42 mA / cm 2 and an efficiency of 1.29%. The area of this device is 0.07 cm2. In order to get a deep understanding of the electrical properties of the heterojunction, capacitance-voltage and current-voltage-temperature measurements have been made. A compact electrical equivalent circuit has been used to describe the dark current-voltage characteristics. It is based on the combination of two exponential mechanisms, shunt and series resistances and space-charge limited current terms. From the temperature dependence of the extracted parameters we can obtain the limiting conduction mechanism. We found that the polymeric layer limits the current not only at low voltages, through Multi-Tunneling Capture Emission, but also at high voltages, through series resistance and Space-Charge Limited Current. On the other hand, the Silicon wafer limits the current at medium voltages, through the diffusion mechanism. In addition, the model is useful to estimate the open circuit voltage and built in voltage of the solar cell using only dark current voltage measurements.

Extraction of poly (3-hexylthiophene) (P3HT) properties from dark current voltage characteristics in a P3HT/n-crystalline-silicon solar cell

The dark current-voltage characteristics of poly (3-hexylthiophene) (P3HT)/n-type crystalline silicon solar cells were analyzed using an electrical equivalent circuit. We found that without illumination transport occurs due to hopping between localized states at the P3HT/silicon interface not only at low voltages, through multitunneling capture emission, but also at medium voltages, through tunneling-enhanced recombination. At high voltages the current is limited by series resistance and space-charge limited mechanisms. At low reverse voltages the current is limited by shunt resistance. From the temperature dependence of the equivalent circuit’s fitting parameters, we were able to estimate some physical parameters of the P3HT layer, namely the electron affinity, the charge carrier concentration and the characteristic temperature of the exponential trap distribution. The extracted P3HT values are in good agreement with previously reported values obtained using different methods but our approach takes into account that the P3HT layer is in a solar cell.

Fabrication and characterization of amorphous Si/crystalline Si heterojunction devices for photovoltaic applications

Heterojunction diode and heterojunction photovoltaic cell structures are fabricated with (n+)a-Si/(i)a-Si and rf-sputtered indium-tin-oxide/Al films deposited on p-type crystalline Si using a plasma-enhanced chemical vapor deposition cluster tool system. Dark current–voltage characteristics of the heterojunction diodes are used to determine the carrier transport mechanisms. Experimental results showed the current is recombination-dominated at low forward bias (VA<0.25 V at 27 °C), multitunneling capture emission (MTCE)-dominated at medium bias (0.25<VA<0.45 at 27 °C), a combination of diffusion or thermionic emission and MTCE at high bias, and series resistance limited at very high bias ranges. Measurement results show that the boundary between each region is not fixed and is strongly dependent on temperature. High-frequency capacitance–voltage measurement has been used to extract band diagram parameters of the heterojunction diodes. Measured parameters, built-in potential, and conduction and valence band offsets, have been verified by Analysis of Microelectronic and Photonic Structures numerical device simulator. External quantum efficiency has been measured on photovoltaic cells and used for calculation of short-circuit current under AM1.5 illumination.

Annealing effects on the conduction mechanisms of p+-amorphous- Si0.8C0.2:H/n-crystalline-Si diodes

P + -type hydrogenated amorphous silicon–carbon (a-Si1−xCx:H) on n-type crystalline silicon (c-Si) heterojunction diodes were fabricated and characterized electrically. The effects of thermal annealing on the electrical transport properties of a-Si0.8C0.2:H/c-Si diodes were investigated by measuring their current–voltage characteristics. From the dark current–voltage characteristics measured at different temperatures (298–373 K), transport mechanisms were analyzed in detail. Two carrier transport mechanisms were found to be the origin of forward current. At low bias voltage and temperatures above 320 K as-deposited diodes are dominated by recombination currents on the amorphous side of the space charge region whereas annealed diodes are mainly dominated by diffusion mechanisms. In contrast, at temperatures below 320 K, both types of diodes are mainly dominated by multitunneling capture emission. At higher voltages, the current becomes space charge limited for both diodes throughout the temperature range studied.

Carrier Transport Mechanisms of a-GaAs/ n-Si Heterojunctions

Heterojunctions have been fabricated of p-type amorphous gallium arsenide (a-GaAs) thin films onto n-type silicon (n-Si) single crystals using thermal evaporation method. Current density-voltage and capacitance-voltage measurements have been performed to determine the electrical properties of the structures. Rectifying current involves tunneling and is explained by a multi-tunneling capture-emission model. The reverse current is limited by the carrier generation process. The capacitance-voltage behavior indicates an abrupt interface with a main-band discontinuity of 0.25 eV occurs in the valance band.

Transport mechanisms and photovoltaic characterizations of spray-deposited of CdS on InP in heterojunction devices

n-CdS/p-InP heterojunctions have been fabricated by deposition of n-CdS thin films using a spray pyrolysis technique onto p-type InP〈100〉. Current density–voltage and capacitance–voltage measurements were performed to determine the electrical properties of the structures. The forward current involves tunneling and is explained by a multi-tunneling capture-emission model. The reverse current is limited by the carrier generation process. The capacitance–voltage behavior indicates an abrupt interface with a main-band discontinuity of 0.76 eV occurs in the valence band. Effect of InP doping density on photovoltaic parameters has also been investigated at different light intensities. The devices exhibit a maximum power conversion efficiency (up to 12%) using an optimum doping density of 1×10 15 cm −3.

Analysis of conduction mechanisms in annealed n-Si1−xCx:H/p-crystalline Si heterojunction diodes for different doping concentrations

We fabricated and characterized annealed n-type amorphous Si1−xCx on p-type crystalline silicon heterojunction diodes with three different base doping concentrations: NA∼1016, 1018, and 1020 cm−3. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that the diodes with low doping concentrations (1016 cm−3) are ideal, because the phosphorous slightly diffuses into the crystalline silicon, whereas diodes with higher doping concentrations (1018–1020 cm−3) are dominated by multitunneling capture emission. The increase in the base acceptor doping concentration also causes excess current over the ideal diode current at low forward bias and an increase in the leakage reverse current.

Electrical properties of a-GaAs/c-Si (p) heterojunctions

Heterojunctions have been fabricated by deposition of amorphous gallium arsenide on p-type crystalline silicon. C( V) and J( V) measurements were performed to determine electrical properties of these structures. Rectifying properties have been obtained, and capacitance-voltage behavior indicates an abrupt interface with a main discontinuity in the valence bands. The forward current involves tunneling and is successfully explained by a multi-tunneling capture-emission model. The reverse current is probably limited by the generation process.

Characterization of boron doped μc-SiC/c-Si heterojunction solar cells

The carrier transport mechanisms and interface physics of boron doped microcrystalline silicon carbide (μc-SiC)/crystalline silicon (c-Si) heterostructures have been studied. The sample structure is p-type μc-SiC/c-Si with the window side electrode. A series of systematic measurements on junction characteristics and photovoltaic properties of the heterojunction solar cells are performed. The results of analysis show that an abrupt heterojunction model and a multi-tunneling capture-emission transport mechanism are found to be valid as evidenced by I = I 0exp(AV) relationships where A is a temperature-independent constant. It has been found out that the direct deposition of μc-SiC:H on the c-Si cause the poor junction properties, probably due to high power plasma bombardments, as evidenced by fairly high shunt currents and poor fill factors. A substantial improvement of the junction quality has been achieved by a low power plasma deposition of less than 50 Å thick a-SiC:H as a buffer layer and an AM1 efficiency more than 13.5% solar cell have been fabricated utilizing a low cost polysilicon and a plasma deposited very thin μc-SiC:H. We also discuss the photo-induced degradation in μc-SiC:H thin films and its effects on the devices.

The effect of the gap DOS in a-Si on the properties on the a-Si/c-Si heterojunction

Based on a new gap DOS model, the effect of the gap DOS in a-Si on the properties of the N a-Si/P c-Si heterojunction is studied. The peak value of acceptor-like deep states make electrostatic potential profile shift, and the low frequency C-V curve has a single minimum value. The multitunneling capture-emission model is used and a multi-tunneling recombination model is proposed to explain the forward J-V characteristics.

Electrical properties of n-amorphous/p-crystalline silicon heterojunctions

We have measured C-V characteristics and temperature dependence of J-V characteristics of undoped hydrogenated amorphous silicon (a-Si:H) heterojunctions formed on p-type crystalline silicon ( p c-Si) substrates with different resistivities. It has been found that an abrupt heterojunction model is valid for a-Si:H/p c-Si heterojunctions, and the electron affinity of a-Si:H has been estimated as 3.93±0.07 eV from C-V characteristics. The forward current of all the junctions studied shows voltage and temperature dependence expressed as exp(−ΔEa f/kT) exp(AV), where ΔEa f and A are constants independent of voltage and temperature, being successfully explained by a multitunneling capture-emission model. The reverse current is proportional to exp(−ΔEar/kT)(VD−V)1/2, where VD is the diffusion voltage and ΔEar is a constant. This current is probably limited by generation-recombination process.