Analysis of a non-uniformly doped MPN silicon Schottky barrier solar cell

An analysis of the solar cell properties of an AlPN Si Schottky diode has been made assuming a generalised impurity distribution in the P-type doped layer. The effect of the image force potential on the effective barrier height is considered. The effect of a constant built-in feld in the substrate has also been included in the analysis. Important solar cell parameters such as open circuit voltage (V_oc), short-circuit current (I_sc) and fill factor (FF) have been calculated as a function of the impurity concentration in the doped layer and its thickness. It is shown that the efficiency η depends strongly on the nature of the impurity profile, being highest for the Gaussian and lowest for the exponential. Finally, the presence of a built-in field (Eb = 4.5 V/cm) in the substrate is found to increase the resultant η which is about 6–8% higher than that of a diode without the drift field.     

Fabrication technology of stable Schottky barrier gates for gallium arsenide MESFETS

An improved structure for stable GaAs Schottky gates formed by the lift-off technique is described. Schottky gates made by the lift-off process often have an interfacial layer and resulting MESFETS have unsatisfactory characteristics. To obtain a transfer characteristic unaffected by the interfacial layer, using rhodium for the contacts is effective. In the case of nickel Schottky contact with interfacial layer, MESFET transfer characteristics are improved by annealing the contact. The Ni film for the contacts consumes GaAs corresponding to its thickness during alloying. To reduce the consumption of GaAs, a thin Ni film should be used.     

An improved analysis of the Schottky barrier solar cell

By discarding the depletion approximation and the assumption of constant quasi-Fermi levels, the actual shape of these levels has been computed in a Schottky barrier solar cell, we believe for the first time. Additional improvements of the theory deal with (a) the dependence of the barrier height on the density of interfacial states, (b) with the various potential drops in the cell, and (c) the allowance for the communication between the metal and interfacial surface states in the treatment of recombination through the interfacial states. The main new results is that a universal model is now available on which one can show quantitatively that:- (1) The quasi-Fermi levels have extrema separated by more than the output voltage and separated by a distance of the order of 1 μm. (2) Thin substrates (～ 4μm) give rise to large hole diffusion currents near the ohmic back contact which is modelled by assuming infinite surface recombination velocity. It replaces as the major loss mechanism the recombination in the semiconductor which dominates for thick (～ 200 μm) substrates. The present author's recent results concerning the reduction in the potential developed across the interfacial surface layer when the device is illuminated has been confirmed and developed. The application in this paper is to n-type cells, but p-type cells can be handled similarly. A preliminary report of the present work is in print.     

Stable tantalum silicide Schottky barrier on n-type gallium arsenide evaluated at elevated temperatures

The high-temperature stability of sputtered tantalum silicide contacts on gallium arsenide has been evaluated. Diodes consisting of Ta silicide on epitaxial n (1.9 × 1017 cm?3; 0.23 ?m/n+ GaAs substrate were annealed at temperatures from 375°C to 800°C. Result show that the ideality factor, barrier height and reverse breakdown voltage remain stable at value of 1.1, 0.79 V and 9 V, respectively. MESFETs with Ta silicide gates exhibited similar drain current/voltage characteristics as conventional Cr/Au gate devices.     

Gallium arsenide dual Schottky barrier diodes

Gallium arsenide diodes were made which had Schottky-barriers for both contacts. Devices which were too thick for space change reach-through to occur at breakdown showed microwave oscillations, while thin diodes did not oscillate. Additionally, the structures could be distinguished on the basis of the noise accompanying breakdown. The performance was analysed in terms of transistor theory in which there is avalanche multiplication in the collector space charge region. It was concluded that there is a smooth transition between the reachthrough breakdown characteristic of the BARITT and true avalanche breakdown. The nature of the breakdown depends on the base width and the emitter efficiency.     

Schottky barriers on p-type gallium arsenide prepared by molecular beam epitaxy

Rectifying contacts have been made by depositing epitaxial films of aluminium and antimony on p-type gallium arsenide by molecular beam epitaxy. The I/V and C/V characteristics were almost ideal. The barrier heights determined from the 1/V characteristics were 0.64 eV for aluminium and 0.66 eV for antimony. The significance of these results is briefly discussed.     

Negative differential resistivity of a nonideal Schottky barrier based on indium arsenide

The process of elastic tunneling is investigated theoretically and experimentally for MOS structures with Schottky barriers based on p ⁺-InAs. At liquid-helium temperatures, currentvoltage characteristics are obtained with negative differential resistivity segments. Using the quasiclassical approximation, analytic expressions are obtained for the tunneling transmission coefficient with carrier conversion. It is shown that the decreasing segments of the currentvoltage characteristic are connected with participation in the tunneling process of high-lying quantum-well levels in the n-channel. The calculated current-voltage characteristics are in good agreement with the results of experiment. Fiz. Tekh. Poluprovodn. 31 370–376 (March 1997)     

The potentialities of silicon and gallium arsenide solar batteries

The theory of the spectral response of a p-n junction solar battery unit is given, and detailed comparisons are made of the expected performance of silicon and gallium arsenide units. Data now available on absorption in GaAs show that it is a particularly favourable material because it has a very steep absorption edge. The low effective masses and correspondingly low densities of states in GaAs are also advantageous.

The efficiency of practical Si units is unlikely to exceed 15 per cent even with intensive development, whereas GaAs units with efficiencies of about 20 per cent should be achievable in the near future and the performance may ultimately approach 25 per cent, even if (as assumed) the carrier lifetimes stay well below those for Si.     

Schottky barrier height variation with metallurgical reactions in aluminum-titanium-gallium arsenide contacts

The behaviour of A1/Ti/n-GaAs Schottky contacts under heat treatment at around 400°C have been studied for their application to GaAs MESFETs. Barrier heights have been determined using both I-V and C-V measurements as a function of heat treatment time. Reaction products due to heat treatment have been studied by X-ray diffraction and Auger electron spectroscopy. The A1/Ti/n-GaAs Schottky contacts barrier height shows an $\text{“N”-}\text{shaped}$ variation in response to heat treatment time. A marked reduction in barrier height follows a slight increase at the initial annealing stage. The barrier height then takes a minimum value. The slight increase in barrier height at the initial stage in the heat treatment is brought about by a Ti and GaAs reaction. The marked barrier height reduction is closely correlated to formation of the compound Al₃Ti. The increase in barrier height observed after the reduction can be explained in terms of GaAlAs formation at the metal-semiconductor interface. This seems to indicate that heat treatment is essential for application of Al/Tin-GaAs Schottky contacts to practical devices.     

Deposition and characterization of gallium arsenide films for solar cells applications

Gallium arsenide films have been deposited on tungsten-coated graphite substrates by the reaction of gallium, hydrogen chloride, and arsine in a hydrogen flow. It has been found that the presence of hydrogen chloride in the gallium monochloride-arsenic mixture is essential in obtaining large-area gallium arsenide films with sufficiently good microstructure suitable for photovoltaic devices. Schottky-barrier and MOS structures have been prepared from n-GaAs/n⁺-GaAs films on tungsten/graphite substrates, and their electrical properties investigated. Under AM1 conditions, MOS structures of 9-cm²area have conversion efficiencies of up to 4.4 percent (without antireflection (AR) coating, would be about 6.7 percent with proper coating), and the conversion efficiencies can be increased to 5 percent (without AR coating, would be about 7.5 percent with proper coating) by using a thin gallium arsenide-phosphide film at the surface of gallium arsenide.     

Breakdown stability of gold, aluminum, and tungsten Schottky barriers on gallium arsenide

The stability of the breakdown voltage of Au, Al, and W Schottky barrier diodes fabricated on n-type GaAs has been evaluated. The nearly ideal breakdown voltage of Au diodes was found to degrade upon annealing, even at below 150°C. The breakdown voltage of the as-deposited Al diodes was half that of the Au diodes, and degraded upon annealing at above 450°C. Secondary ion mass spectroscopy (SIMS) measurements revealed penetration of Al into the GaAs. The breakdown voltage of W diodes increased to equal that of the Au diodes upon annealing above 400°C and remained stable at up to 550°C. No interdiffusion at the W/GaAs interface was observed.     

Cost trade between multijunction,gallium arsenide and silicon solar cells

Multijunction (MJ),¹ gallium arsenide (GaAs) and silicon (Si) solar cells have respective test efficiencies of approximately 24%, 18.5% and 14.8%. Multijunction and gallium arsenide solar cells weigh more than silicon solar cells and cost approximately five times as much per unit power at the cell level.² A trade is performed for the Tropical Rainfall Measuring Mission (TRMM) spacecraft to determine which of these cell types would have offered an overall performance and price advantage to the spacecraft. A trade is also performed for the multijunction cells under the assumption that they will cost over ten times that of silicon cells at the cell level. The trade shows that the TRMM project, less the cost of the instrument, ground systems and mission operations, would spend approximately $552 000 per kilogram to launch and support

1 The term ‘support’ means to provide the scientific equipment (‘science’) with environmental protection from the space environ ment, to keep it at acceptable temperatures and to send the data it produces to Earth in readable form, i.e. the services provided by the spacecraft.

science in the case of the spacecraft equipped with silicon solar cells. If these cells are exch anged for gallium arsenide solar cells, an additional 31 kg of science can be launched and serviced at a price of approximately $90 000 per kilogram. The 31 kg array weight reduction is shown to derive from the smaller area of the array and hence reductio ns in the weight of the array substrate and supporting structure. If the silicon solar cells are changed out for multijunction solar cells, an additional 45 kg of science above the silicon baseline can be launched and supported at a price of approximately $58 000 per kilogram. The trade shows that even if the multijunction cells are priced over ten times that of silicon cells, a price that is much higher than projected, the additional 45 kg of science are launched and serviced at $180 000 per kilogram. This is still much less than the original $552 000 per kilogram to launch and service the science. Data and qualitative factors are presented to show that these figures are subject to a great deal of uncertainty. Nonetheless, the benefit of the higher efficiency solar cells for TRMM is far greater than the uncertainties in the analysis. © This article is a US Government work and, as such, is in the public domain in the United States of America     

An ultrahigh speed modulated barrier photodiode made on P-type gallium arsenide substrates

We demonstrate a modulated barrier photodiode (MBP) grown by molecular beam epitaxy on p-type gallium arsenide substrates. This device has an ultrahigh response speed with a rise time <30psec and a full width at half maximum <50psec. A DC optical gain of 90 has also been measured for an incident power in the nanowatt range.     

Photoreflectance characterization of gallium arsenide

A novel method of noncontact and nondestructive characterization by photoreflectance spectroscopy is validated on bulk and epitaxial GaAs single crystals with different levels of doping. The method is used to simultaneously determine the surface electric field and free-carrier concentration. Close agreement is observed between photoreflectance-spectroscopy and Hall-effect measurements.     

High-temperature irradiation of gallium arsenide

Deep level transient spectrocopy was used to investigate the introduction of P2 and P3 centers into n-type epitaxial layers of GaAs as a result of exposure to 4-MeV electrons in the temperature range 380–550 °C. It is shown that the rate at which the centers are introduced into the layers is independent of temperature in this range. The P2 center concentration is proportional to D ^0.7, whereas for the P3 center this function is D ^0.5, where D is the electron dose. Fiz. Tekh. Poluprovodn. 31, 1234–1235 (October 1997)     

Broadband enhancement of coaxial heterogeneous gallium arsenide single‐nanowire solar cells

Coaxial gallium arsenide single‐nanowire solar cells with multiple electrically and optically functional nanoshells are presented in this paper. Both optical absorption and light‐conversion characteristics are extensively examined by performing a comprehensive device‐oriented simulation. It is found that a window layer with a large semiconductor bandgap is necessary for the nanowire gallium arsenide solar cells, which allow internal quantum efficiency ~100% in ~75% of the absorption band of gallium arsenide. Results also reveal the role of nanofocusing effect in enhancing the performance of nanowire devices that show both absorption and external quantum efficiencies over 100% under resonances. A dielectric cladding shell is introduced and optimized, which enhances the nanofocusing effect and leads to extraordinary enhancement of both absorption and light‐conversion capabilities in a very broad band. This design contributes a short‐circuit current density increased by 2.4 times and an open‐circuit voltage over 1.1 V. Copyright © 2014 John Wiley & Sons, Ltd.     

Breakdown characteristics of gallium arsenide

This paper presents a comparison of the experimentally measured breakdown characteristics of high-purity gallium arsenide epitaxial layers with the theoretical calculations reported in the literature. The experimental data has been obtained by forming gold Schottky barrier diodes on material grown by using two different liquid phase epitaxial growth techniques. Good agreement is observed between the measured data and the breakdown voltages calculated by Lee and Sze[4] for