H. Thomas

Transport properties of GaAs layers grown by molecular beam epitaxy at low temperature and the effects of annealing

The effects of growth temperature and subsequent annealing temperatures on the electrical properties of the low temperature (LT) grown GaAs have been investigated. It was found that the resistivity of the as‐grown LT‐GaAs layer increased with increasing growth temperature, but was accompanied by a reduction of breakdown voltage over the same temperature range. Thermal annealing of the samples caused the resistivity to rise exponentially with increasing annealing temperature TA, giving an activation energy of EA=2.1 eV. The transport of the LT‐GaAs layers grown at Tg≤250 °C was found to be dominated by hopping conduction in the entire measurement temperature range (100–300 K), but following annealing at TA≳500 °C, the resistivity‐temperature dependence gave an activation energy of ∼0.7 eV. The breakdown voltage VBD, for as‐grown LT‐GaAs was enhanced on lowering the measurement temperature, but conversely, decreased over the same temperature range following annealing at TA≳500 °C. The hopping conduction bet...

Investigation of the negative peak in photoinduced transient spectra of semi‐insulating gallium arsenide

A common observation in the photoinduced transient (PITS) spectra of semi‐insulating GaAs is the appearance of a negative peak, which is anomalous in that both electron and hole traps should give rise to positive peaks. In this paper, it is shown that the negative peak can be explained in terms of charge exchange with the GaAs surface and only occurs in material which displays particular types of current‐voltage and current‐temperature characteristics. The dependence of this peak on the processing effects of surface passivation, etching, and baking and polishing has been investigated and its sensitivity to variations in incident light intensity is demonstrated. A new variation of PITS, namely gated‐PITS has been employed. This technique suppresses the negative peak in the spectrum, allowing transients corresponding to emission from EL2 to be detected in particular undoped liquid encapsulated Czochralski GaAs substrates for the first time.

Thermal annealing effect on low temperature molecular beam epitaxy grown GaAs: Arsenic precipitation and the change of resistivity

The post‐growth annealing effects on the electrical properties of low temperature (LT‐) GaAs grown by molecular beam epitaxy have been investigated. It was found that the resistivity of the LT‐GaAs layer increased exponentially with annealing temperature TA, resulting in an activation energy of 2.1 eV. This activation energy is related to the activation energy of arsenic precipitation. Based on hopping conduction theory, an As cluster density NT, has been estimated from the resistivities of the LT‐GaAs layers. The change of density of arsenic clusters with TA, was found to be of the form NT=NT0 exp(−T/T0), in agreement with values obtained by transmission electron microscopy measurements. The breakdown voltage of the LT‐GaAs layer remained almost unchanged as TA was increased up to 650 °C, but the breakdown characteristic became soft. The formation of As clusters is held responsible for the soft breakdown of the LT‐GaAs layer after annealing.

The effect of growth temperature on the electrical properties of AlInAs/InP grown by molecular beam epitaxy and metal‐organic chemical‐vapor deposition

Al0.48In0.52As layers grown on n+‐InP substrates by molecular beam epitaxy (MBE) and metal‐organic chemical‐vapor deposition (MOCVD), as a function of substrate temperature, have been characterized by current‐voltage‐temperature, capacitance‐voltage, deep‐level transient spectroscopy, and admittance spectroscopy measurements. It was found that for diodes formed on MOCVD‐AlInAs the current in forward bias is dominated by thermionic emission and the reverse current by recombination through the residual midgap states; whereas, for MBE‐AlInAs diodes, the respective currents are dominated by defect‐assisted tunneling at low forward and reverse biases. Schottky barrier heights were found to decrease with decreasing growth temperature. Three defect levels E1, E2, and E3 were observed in both material systems, and their densities were found to increase rapidly from ∼1012 to ∼1016 cm−3 as the growth temperature decreased from 740 to 500 °C. The decrease of barrier height and the appearance of the defect‐assisted t...

Transport properties of indium tin oxide/p‐InP structures

I‐V properties of indium tin oxide (ITO)/p‐InP solar cell structures measured at various temperatures show that the conduction mechanism is dominated by tunneling at low forward bias, and by thermionic emission at high forward bias. An increase of barrier height of 200–300 meV was found for all ITO/InP diodes compared with Au/InP Schottky diodes. Donorlike defects were found to be responsible for the increase of barrier height, and to cause the defect‐assisted tunneling conduction. The experimental results support the model of a buried n+/p junction with the n+ layer induced by the sputter deposition of ITO.

The influence of indium tin oxide deposition on the transport properties at InP junctions

The influence of the deposition of indium tin oxide (ITO) on the electrical properties at n- and p-InP junctions have been investigated by current-voltage-temperature and capacitance-voltage measurements. It was found that the formation of the ITO layer on n-type InP substrates causes the reduction of the barrier height and subsequently forms an ohmic contact. The ITO layer on p-type substrates increases the barrier height by 200–300 meV, and causes a defect-assisted tunneling at low forward bias. The results, therefore, can be explained by the introduction of process induced donor-like defects, with the formation of a thin n+-layer in the near-surface, decreasing the barrier height for n-substrates and increasing the barrier height for p-substrates. These results support the buried n+/p-junction model for ITO/p-InP solar cell structures.

LT-GaAs with high breakdown strength at low temperature for power MISFET applications

Low temperature grown GaAs has been fabricated containing a limited amount of excess arsenic. The material has a low conductivity in the order of 100KΩ cm, due to hopping in a deep donor band. This σ-LT-GaAs was grown reproducibly by using the lattice mismatch as the primary parameter for substrate temperature calibration. Breakdown fields, in the order of 100kV/cm, are observed for planar structures and increased at low measurement emperatures. Low hopping conductivity and high breakdown field are also observed in the lossy dielectric metal-insulator-semiconductor field-effect transistor device using σ-LT-GaAs as a surface layer. The record radio frequency power density of 4.0W/mm at 77K is extracted from the dc output characteristics.

Annealing of irradiation damage in epitaxial InP homojunction solar cells

Changes in the performance of epitaxial InP homojunction solar cells after irradiation and annealing are discussed, for 1 MeV electron and 1 MeV proton irradiation. Material samples have been investigated in order to characterise the irradiation induced defects. Admittance spectroscopy has identified a defect, HD1, which cannot be observed by DLTS and which is believed to influence carrier concentration in the p-type material. The relationship of the defect characterisation to thermal annealing of the cells, both at room and elevated temperature, is considered.<<ETX>>

Minority carrier traps in epitaxial gallium arsenide phosphide

The optical injection of minority carriers in the depletion region of a reverse biased Schottky barrier has permitted the study of deep levels in vapour phase epitaxial n-type GaAsP (Te) structures. The deep level transient spectroscopy (DLTS) experiments have yielded two hole traps at (Ev + 0.60)eV and (Ev + O.36)eV. Additional subsidiary transient capacitance, photocurrent and capture experiments are analysed, the details of which give electron and hole capture cross-sections, trap densities and activation energies. Derived values of minority carrier lifetime (~1 ns) agree with experimental values and with injection-level dependent LED efficiency.

Comparison of the radiation resistance of electron irradiated indium phosphide grown by metal‐organic chemical‐vapor deposition and liquid encapsulated Czochralski

Electrical characterization has been carried out on electron irradiated InP grown by metal‐organic chemical‐vapor deposition (MOCVD) and liquid encapsulated Czochralski (LEC), through I–V (–T), C–V, deep level transient spectroscopy (DLTS) and admittance spectroscopy measurements and the resistance to electron radiation for these two materials has been compared. It was found that MOCVD‐InP was more resistant to electron radiation than LEC‐InP, as demonstrated by the lower carrier removal rate and change of series resistance in the MOCVD‐InP diodes as a result of electron radiation. The introduction rates for the dominant hole defects H3 and H4 and for additional electron defect states were found to be similar for both materials, but were insufficient to explain the degree of degradation of solar cell efficiency incurred by these known defects. A new defect, HD1, has been found to be responsible for the high carrier removal rate and the introduction of a large series resistance which accounts for the diffe...