Multistep Tunneling Research Articles

Mechanism of carrier transport in highly efficient solar cells having indium tin oxide/Si junctions

The carrier transport mechanism of the Si solar cells having n-Si/indium tin oxide (ITO) junctions has been studied by use of the current-voltage and capacitance-voltage measurements and x-ray photoelectron spectroscopy. An 11-Å-thick nonstoichiometric Si oxide layer is formed when ITO is deposited by spray pyrolysis on a Si electrode etched with hydrofluoric acid. In this case, the tunneling probability of majority carriers through the oxide layer is high, and the thermionic emission current over the energy barrier in Si takes a dominant part of the dark current. On the other hand, for a Si electrode where a Si oxide layer is intentionally interposed between ITO and Si, the thermionic emission current is suppressed, and trap-assisted multistep tunneling through the depletion layer becomes dominant. By making a mat-structure treatment on the Si surface, a solar energy conversion efficiency of 13% and the photocurrent density of 42.5 mA cm−2 were attained under AM 1 100 mW cm−2 illumination.

Observation of recombination center-assisted tunneling current in Al(Cu)-penetrated PtSi Schottky barrier diodes

The electrical transport characteristics of Al(Cu)-penetrated PtSi Schottky barrier diodes were investigated. It has been observed for the first time that diodes with Al(Cu) penetration show a multistep recombination center-assisted tunneling current in the forward direction instead of a thermionic emission current over the Al(Cu) alloy barrier. According to this electron transport mechanism an electron tunnels through a series of closely packed local recombination centers in the depletion region to traverse from the semiconductor to the metal side. When the penetrated diodes are reverse biased, Zener tunneling becomes the dominant conduction mechanism in the reverse direction. Al(Cu) penetration in PtSi Schottky barrier diodes produces high leakage currents and unpredictable turn-on voltage characteristics which result in various design problems in logic circuits.

Electrical properties of GaAs homojunctions grown by MOCVD on GaAs and Si substrates

Several techniques, including DLTS, I- V- T, C- V- T, C- T and photoreflectance spectroscopy were used to investigate the electrical properties of GaAs multilayer heteroface p- n junctions epitaxially grown by MOCVD on GaAs or Si substrates in order to study the influence of heteroepitaxy on device properties. A deep level (EL2) was detected for both GaAs/GaAs and GaAs/Si structures. For GaAs/Si, a continuous distribution of trap levels was also observed which could be ascribed to misfit defects. I- V- T data revealed the saturation current of GaAs/Si diodes to be four orders in amplitude higher than for GaAs/GaAs ones. Different temperature dependences were found in saturation current density, barrier height and capacitance between the two structures. A multistep tunneling model is utilized in explaining the results for GaAs/Si. C- T measurements indicated that the free carriers are not frozen-out for GaAs/Si at the observed freeze-out temperature of GaAs/GaAs. Defect-induced bandgap tailing might be responsible for this effect.

Current transport mechanisms in epitaxial CdS/CdTe heterojunctions

Epitaxial n-CdS/p-CdTe heterojunctions have been fabricated by the vacuum evaporation of CdS onto phosphorus doped {⦶111} B CdTe substrates. Analysis of the current-voltage and capacitance-voltage characteristics suggests that electrical transport across the junction is dominated by a multi-step tunneling process. Although open circuit voltage and short circuit current values were high, the overall photovoltaic conversion efficiency of the devices was restricted to ≈ 6%, because of the relatively high resistivity of the CdTe substrates.

Some electrical and photovoltaic properties of poly-Si/SnO 2 heterojunction

Thin films of antimony-doped SnO 2 deposited on polycrystalline silicon formed a heterojunction with SIS (“Semiconductor-Insulator-Semiconductor”) structure, since an interfacial layer of SiO x between Si and SnO 2 was always present. Thin films of SnO 2 were formed by a new modification of the CVD (Chemical Vapour Deposition) method. The deposition parameters and the doping by antimony (Sb) were optimized in order to obtain SnO 2 films of low sheet resistance, while maintaining high solar transmittance. DC current-voltage, (I–V) characteristics of the heterojunctions were measured in the dark at different temperatures and under sunlight, together with the output I-V curves. The experimental results indicate that multi-step tunnelling through the silicon barrier might be the predominant current transport mechanism in the dark, at least in the temperature range between 290 and 360 K. It is suggested that photovoltaic properties of these n-SnO 2/ n-Si (poly) heterojunctions could be improved by applying a metal grid front contact to SnO 2 and by using thicker SnO 2 films with lower sheet resistances.

Electrical Properties of Mesa Diodes on Epitaxial GaAs/Si

AbstractDeep level transient spectroscopy (DLTS), current-voltage- temperature (I-V-T), capacitance-voltage-temperature (C-V-T), C-T and photoreflectance spectroscopy (PRS) were used to compare the electrical properties of GaAs p-n junctions epitaxially grown by MOCVD on GaAs and Si substrates. EL2 was detected for both GaAs/GaAs and GaAs/Si structures. For GaAs/Si, a continuous distribution of trap levels was also observed which could be ascribed to misfit defects. I-V-T data revealed the saturation current of GaAs/Si diodes to be four orders in amplitude higher than for GaAs/GaAs ones. Different temperature dependences were found in saturation current density, barrier height and capacitance between the two structures. A multistep tunneling model is utilized in explaining the results for GaAs/Si. C-T measurements indicated the free carriers not to be frozen-out for GaAs/Si at the freeze-out temperature of GaAs/GaAs. A defect- induced bandgap tailing effect might be responsible for this effect, verified by PRS tests.

Current transport in strained n-Si1−xGex/p-Si heterojunction diodes

The measurement and analysis of I-V characteristics from molecular beam epitaxially grown Si0.9Ge0.1/Si p-n diodes are presented. When a diode is forward biased the current transport is mainly injection diffusion of the holes and defect recombination within the depletion region of the n side. At reverse bias, the current comes from tunneling of holes from the top of the p-type valence band to the n-type conduction band, and also to the interface states followed by multistep recombination tunneling via defect states in the depletion region of the n side. The forward voltage drop was found to be much lower than that of Si diodes. Electron irradiation damages have opposite effects on the forward-biased I-V characteristics of Si0.9Ge0.1/Si diodes as compared to the effects observed for Si diodes.

Schottky barrier junctions of hydrogenated amorphous silicon-germanium alloys

The current-transport mechanisms of Au/a-SiGe:H junctions have been investigated. It has been found that electrons are transported by multi-step tunneling through a part of the Schottky barrier, which can be explained by the “diffusion-field-emission” model. The existence of tunneling process has also been confirmed by the temperature dependence of isothermal-capacitance-transient-spectroscopy (ICTS) signals. The bump of DOS in a-SiGe:H has been found to be located at about 0.4 eV below the conduction band edge.

Fabrication and analysis of all-sprayed CuInS 2/ZnO solar cells

CuInS 2/ZnO solar cells of 2% efficiency with V oc = 280 mV, I sc = 13.3 mA cm −2 and FF = 0.38 were prepared by spray pyrolysis. The role of CuInS 2 deposition parameters and the resistivity of CuInS 2 and ZnO films in the formation of efficient junctions was studied. Maximum efficiency was obtained for indium-doped ZnO films with an [In]/[Zn] atomic concentration ratio of 0.03 and CuInS 2 films deposited with 12% excess copper in spray solution. Deposition of an interlayer of CuInS 2 and post-deposition annealing of junctions are essential for obtaining good quality solar cells. Multistep tunnelling and recombination is the most likely mode of carrier transport in CuInS 2/ZnO heterojunctions.

Schottky barrier junctions of hydrogenated amorphous silicon-germanium alloys

The current-transport mechanisms of amorphous Schottky barrier junctions are investigated using both undoped and phosphorus-doped hydrogenated amorphous silicon-germanium alloys (a-Si1−xGex:H) fabricated by rf glow discharges in diode- and triode-reactor systems with and without the dilution of hydrogens. From the experimental results, the forward-current mechanisms for amorphous Schottky barrier junctions are generally discussed and classified into three categories: (1) field emission, (2) diffusion-field emission, and (3) diffusion. The diffusion-field-emission model (multistep tunneling through a part of the Schottky barrier) used for the first time in this work can interpret the current-voltage characteristics and their temperature dependence on amorphous Schottky barrier junctions to a high degree. Moreover, the flow chart proposed here classifies the material quality of amorphous semiconductors according to their junction properties. We concluded that one of the better methods to fabricate high-quality a-Si1−xGex:H for Schottky barrier junctions is the triode-reactor technique using hydrogen-diluted starting-gas materials.

Current transport in p-type CdIn2Te4 Schottky diodes

Schottky diodes were fabricated on p-type CdIn2Te4 using evaporated aluminum dots. The forward and reverse current-voltage characteristics of these Schottky barrier devices were studied as a function of temperature. An analysis of this data allows the determination of the barrier height, the behavior of traps, and the current transport mechanism of the devices. The forward conduction mechanism appears to be a multistep tunneling process. The reverse transport is dominated by a diffusion process. The height of the barrier associated with the junction as revealed by the reverse I-V characteristics is 0.4 eV. The conductivity type of the crystals can be changed from n type to p type in a manner similar to CdTe.

Fabrication and characterization of SnO 2:F/n-Si(poly) solar cells prepared by chemical vapour deposition

SnO 2:F/n-Si(poly) solar cells prepared by a low temperature chemical vapour deposition process exhibit good photovoltaic properties. A thin insulating layer on Si improves the open-circuit voltage (V oc ) and an adherent back layer of Ni, deposited by electroplating, forms a good ohmic back contact. The dark current vs. voltage characteristics reveal that the dominant transport mechanism is multistep tunnelling, and the (capacitance) −2 vs. voltage curve shows that the junction is abrupt. The best cell generates an open-circuit voltage of 0.505 V, a short-circuit current density (active area) of 240 A m −2, a fill factor of 0.61 and an efficiency of 9.2% under 800 W m −2 insolation using an AM 1 filter in a solar simulator. In addition, the relative spectral response is high over a wide range of wavelengths, and in the higher energy region it is better than that of a p-n homojunction silicon solar cell.

Temperature dependence of shunt resistance in photovoltaic devices

Shunt resistance in surface passivated Si solar cells has been experimentally shown to be temperature dependent with a maximum value of 2×107 Ω cm2. Mechanisms responsible for shunt resistance are identified as a combination of multistep tunneling and thermal trapping-detrapping of carriers through the defect states in the space-charge region.

Current–voltage characteristics of n-AlGaAs/p-GaAs heterojunction diodes

To understand the current injection mechanism in AlGaAs/GaAs heterojunction bipolar transistors (HBT’s), current–voltage characteristics of n-AlGaAs/p-GaAs heterojunctions have been studied for varying doping densities and as a function of temperature. The bulk space-charge recombination current studied in heterojunction diodes identical to emitter–base junction of an HBT has been found to be an order of magnitude smaller than that in GaAs homojunction diodes. It approximately varies as A(T) exp 20V where A(T) is a decreasing function of temperature. The nature of this current is similar to the excess current in tunnel diodes and is considered to be due to multistep tunneling followed by recombination. The tunneling nature of the current is established by the observed electroluminescence and the effect of varying doping densities. It is concluded that the surface currents normally dominate the base current and that they largely depend on the fabrication process and device geometries.

SnO 2:F/n-Si and In 2O 3:Sn/n-Si semiconductor/insulator/ semiconductor solar cells

Solar cells comprising chemically deposited SnO 2:F and In 2O 3:Sn, a thin SiO x layer grown in concentrated HNO 3 and n-Si substrates with (100) orientation yielded efficiencies of about 11%. X-ray photoelectron spectroscopy studies of the SnO 2:F films revealed fluorine doping in SnO 2. No trace of SnO or Sn 3O 4 compounds was observed. The amount of fluorine present in the films ([F]/[Sn] ≈ 0.08) was much less than that in the solution ([F]/[Sn] ≈ 1.02). In the In 2O 3:Sn films the tin replaced indium rather than forming an alloy of In 2O 3 and SnO 2. The dark current density versus voltage characteristics revealed the mechanism of transport to be multistep tunnelling. High frequency capacitance-voltage measurements showed that the junction is abrupt with barrier heights of 0.96 eV and 0.93 eV in SnO 2:F/n-Si and In 2O 3:Sn/n-Si heterostructures respectively.

ZnO/CdTe heterojunctions prepared by r.f. sputtering

ZnO/CdTe heterojunctions were prepared by r.f. sputtering of ZnO films onto p-type CdTe single crystals. The ZnO deposition was carried out using a ZnO target and an Ar-H 2 mixture as the sputtering gas. The ZnO films obtained under these conditions show a very good optical transmission (about 90%) between 0.4 and 0.8 μm and a low resistivity. The electrical properties of the ZnO/CdTe heterojunctions were studied by means of current-voltage ( I-V) and capacitance-voltage ( C-V) measurements carried out at different temperatures. The dark I-V characteristics show that a multistep tunnelling mechanism controls the transport of charge carriers across the junction. The presence of interface states was shown to play an important role in the electrical properties of the junction. Tentative measurements of the solar energy conversion efficiency gave a value of about 3.5% without any attempt at optimization of the cell parameters.

Effects of interfacial charge on the electron affinity, work function, and electrical characteristics of thinly oxidized semiconductor-insulator-semiconductor and metal-insulator-semiconductor devices

A phenomenological theory involving interfacial charge is presented to account for the result that the diode constant n at low forward biases might be temperature dependent in some thinly oxidized (< 20 Å) semiconductor-insulator-semiconductor and metal-insulator-semiconductor devices, resulting in parallel I-V curves. Neither multistep tunneling through the semiconductor space-charge region, nor conventional thermionic emission over the barrier, can explain the low-voltage portion of the I-V characteristics. In this paper it is shown that an interfacial charge redistribution occurs with changes in temperature, resulting in changes in the electron affinity of the base semiconductor and in the work function of the contact material. This results in a linear dependence of n on reciprocal temperature. An explanation is presented to account for the large temperature coefficient that has been observed for both the diffusion potential VD and open-circuit photovoltage Voc in these devices. And an explanation of the I-V characteristics in terms of shunt currents is also given.

Influence of semiconductor barrier tunneling on the current-voltage characteristics of tunnel metal-oxide-semiconductor diodes

Current–voltage characteristics have been examined for Al–SiO2–pSi diodes with an interfacial oxide thickness of δ≊20 Å. The diodes were fabricated on 〈100〉 and 〈111〉 oriented substrates with an impurity concentration in the range of NA=1014–1016 cm−3. The results show that for low forward voltages, the diode current is increased with increased NA, but for higher forward voltages, the diode current is decreased as NA is increased. For the diodes examined in this work, the results presented lead to the conclusion that the diode current should be treated as a superposition of multistep tunneling recombination current and injected minority carrier diffusion current. This can explain the observed values of the diode quality factor n. The results also show that the voltage drop across the oxide Vox is increased with increased NA, with the result that the lowering of the minority carrier diode current Jmin is greater than in the usual theory. The conclusion drawn is that the increase in Vox and lowering of Jmin is due to multistep tunneling of majority carriers through the semiconductor barrier.

Diode laser fabrication using proton bombardment of PbTe

Diode lasers, in which the p- n junction was formed by causing type conversion in a p-type PbTe substrate using proton damage, have been made and their performance characterised. Variations of the energy, flux and fluence of the proton bombardment over wide ranges were found to have little or no effect on the diode I-V, C-V or power output characteristics, but such variations did affect the yield of diodes which lased. The C-V characteristics indicated that the p- n junctions were abrupt. The I-V characteristics suggested that the forward conduction current was dominated by a non-radiative multi-step tunnelling process.

Transport mechanisms for Mg/Zn3P2 junctions

The dominant transport mechanisms for Mg/Zn3P2 junctions are shown to depend on the heat treatment in hydrogen of the Zn3P2 prior to Mg deposition. For heat treatments below 300 °C, multistep tunneling via defect centers dominates, whereas for heat treatments between 300 and 500 °C, recombination/generation dominates. Correspondence is observed between the imperfections responsible for the junction transport and previously reported imperfections in bulk Zn3P2 crystals. Enhanced tunneling under solar illumination contributes to the low energy conversion efficiency of thin Mg film Mg/Zn3P2 devices. An estimate of the interface recombination velocity for these junctions yields a value of 1.9×107 cm/sec.