Undoped Semi-insulating GaAs Crystals Research Articles

Effect of the vacancy composition of GaAs single crystals on optical quenching of luminescence through EL2 defects

Optical quenching of luminescence through EL2 defects in single crystals of undoped semi-insulating gallium arsenide is investigated. It is shown that the minimum energy of light photons providing the transition of such defects into the metastable state depends on the vacancy composition of the crystals. It is suggested that the nature of the effect revealed is related to the existence of a set of EL2 defects with different configurations. An optical method is proposed to determine the vacancy composition of undoped semi-insulating GaAs crystals.

An analysis of the shape of a luminescence band induced by transitions of free electrons to carbon atoms in semi-insulating undoped GaAs crystals

The shape of a photoluminescence band observed due to recombination of free electrons at shallow-level acceptors (carbon atoms) in semi-insulating undoped GaAs crystals was analyzed at various temperatures (T=4.8–77 K). It is shown that at low temperatures the shape observed essentially differs from the theoretical one, while at high temperatures theory and experiment agree closely for radiative transitions of free electrons to isolated shallow-level acceptors. The difference between the experimental and theoretical shapes of the photoluminescence band is associated with the broadening of carbon-induced acceptor levels (i.e., with the formation of the acceptor impurity band), resulting from the effect of electric fields of randomly distributed ionized acceptors and donors on “isolated” carbon atoms. Coincidence of the shapes is associated with a considerable increase in the energy of free carriers (to values up to and above the width of the acceptor impurity band).

Influence of heat treatment on luminescence of semi-insulating undoped GaAs crystals

Variations in the spectra of edge-emission photoluminescence of semi-insulating undoped GaAs crystals as a result of heat treatment for 20–90 min at 900°C were studied. It is shown that heat treatment substantially affects (transforms) the excitonic component of the spectrum. The observed changes in the spectra are related to variations in the impurity composition of the crystals studied. These variations are shown to be caused by heat treatment.

Carbon-stimulated increase in the concentration of gallium divacancies in semi-insulating undoped GaAs crystals

It is shown that an increase in carbon content in semi-insulating undoped GaAs crystals leads to a substantial rise of the concentration of gallium divacancies in these crystals. This effect seems to be related to the process of carbon atoms occupying the arsenic vacancies involved in the As-divacancy-Ga-divacancy complex.

Analysis of the Low-Temperature (77 K) Near-Band-Edge Luminescence in Undoped Semi-Insulating GaAs

The dependences of the peak position and of the half-width of the near-band-edge luminescence band in nominally undoped semi-insulating GaAs crystals on the content of residual C and Si shallow impurities are studied at 77 K. It is shown that a rise in the impurity concentration leads to a shift of the peak position to lower energies and to an increase of the half-width. The observed effect is mainly connected with a broadening of the excitonic component of the near-band-edge luminescence band resulting from an increase in the efficiency of radiative recombination of free excitons due to their interaction with C and Si shallow impurities.

Improvement in Semi-Insulating GaAs Material Quality: A Comparative Study of Defects with Deep Levels

Thermally stimulated currents (TSC) spectra and photocurrent (IPC) measurements were used for detection and evaluation of defects with deep levels in undoped semi-insulating (SI) GaAs crystals. Large number of liquid encapsulated Czochralski (LEC) grown materials, produced from late 80's till nowadays, provided from ten various sources were analysed. Deep levels were characterised by a new analytical method-simultaneous multiple peak analysis (SIMPA) of TSC spectra. For each deep trap its unique and reliable signature was determined, comprising activation energy, Ea, electron capture cross section, σn, as well as peak maxima, Tm, and trap's relative and absolute concentrations. It has been found that all measured TSC spectra, even having dramatically different shapes, can be excellently described with a limited set of 11 deep traps. Considerable improvement in quality of the SI GaAs materials produced during last decade was found. It reflects in concentration reduction of most defects and much better defect's distributional uniformity along the same as well as among different wafers, and in similarity of IPC transients.

Analysis of changes in the intensity of intrinsic luminescence after diffusion of copper into semi-insulating undoped gallium arsenide crystals

The effect of copper diffusion into semi-insulating undoped GaAs crystals on the intensity of intrinsic luminescence is analyzed. It is shown that diffusion of copper into semi-insulating undoped GaAs crystals can lead either to an increase or a decrease in the intensity of intrinsic luminescence. Analytic relations, which connect the magnitude and sign of the effect with recombination parameters in these crystals, and also with the intensity of the excitation luminescence, are obtained.

Analysis of Changes in the Intensity of the Intrinsic Luminescence after the Copper Diffusion into Semi-Insulating Undoped GaAs Crystals

An analysis is made of the effect of the copper diffusion into semi-insulating undoped GaAs crystals on the intensity of the intrinsic luminescence. It is shown that the copper diffusion into semi-insulating undoped GaAs crystals could lead both to an increase and to a decrease in the intrinsic luminescence intensity. Analytical expressions connecting the value and the sign of the effect observed with the recombination parameters of crystals pointed and also with the intensity of luminescence excitation are obtained.

A comparative study of deep levels in undoped semi-insulating gallium arsenide wafers using thermally stimulated current spectra

Thermally stimulated current (TSC) spectra and photocurrent (Ipc) measurements are used for determination and evaluation of deep levels, as well as for the comparison of their distributional uniformity in undoped semi-insulating (SI) GaAs crystals. Liquid-encapsulated Czochralski-grown (LEC) GaAs wafers from various sources were analysed. It was found that crystal growth and post-growth treatment of samples from each producer give characteristic and distinctive TSC spectra. The samples from different positions at wafers show good uniformity of deep-level distribution for some producers, whereas the uniformity is quite poor for the others. Some characteristic peaks appear in spectra of all crystals, suggesting either the presence of native defects or some of the most common contaminants. The results obtained are compared with those in previous reports. Furthermore, a considerable difference in photocurrent values after 100 s illumination has been found, indicating large free carrier lifetime scattering for samples of different origins. This study shows how, by using quick and simple TSC and Ipc measurements, the quality of the material and uniformity of the distribution of defects with deep levels in the forbidden energy gap can be assessed.

Dominant Midgap Levels in the Compensation Mechanism in GaAs

AbstractProperties of midgap levels in n-type GaAs crystals were studied by using trap density spectroscopy (TDS), capacitance-voltage and near infrared (NIR) absorption measurements. The TDS analysis for various n-type samples showed that the concentrations of the midgap level were from 2.0 × 1016 cm−3 to 3.4 × 1016 cm−3. However, those of EL2 in the same crystals were found to be from 1.1 × 1016 cm−3 to 1.3 × 1016 cm−3 by NIR absorption measurements at room temperature. On the other hand, in undoped semi-insulating GaAs crystals with carbon concentrations ranging from 6 × 1014 cm−3 to 2.4 × 1016 cm−3, the densities of the ionized EL2 determined by NIR absorption were only about 30 % of those of carbon acceptor determined by the localized vibrational mode absorption at room temperature. These differences suggest the presence of another midgap donor which does not give NIR absorption. The concentrations of this trap were estimated to be 0.8 to 1.6 times those of EL2 by decomposition of the TDS spectra into their components.

Investigation of changes of As precipitates in semi-insulating GaAs crystals at several temperatures by infrared light scattering tomography

Thermal behavior of arsenic precipitates in undoped semi-insulating GaAs crystals has been investigated. Crystals heat-treated at a temperature between 500 and 1150°C were observed by infrared light scattering tomography, which is powerful technique capable of detecting the arsenic precipitates. The arsenic particles on the dislocations became large increased in scattering intensity with temperatures from 800 to 1000°C. A dramatic change occurred above 1100°C: the images disappeared over the whole area. This disappearance means that the arsenic precipitates dissolves into the GaAs crystal. Using the samples in which the excess arsenic was solid-soluted, the same experiments were repeated. At 800–1000°C, the scattering images of the dislocations again appeared. Below 600°C, new images, consisting of misty zones of scattering, were observed in the dislocation-free areas. This suggests that the excess arsenic condenses like mist over the limit of saturation solubility.

Properties of semi-insulating GaAs grown by a vertical molten zone method

Electrical and optical measurements are reported for samples from two undoped semi-insulating GaAs crystals grown by a vertical molten zone method. The electrical data, taken over the range 290–420 K, included results for samples from both crystals that were so close to intrinsic as to require an ambipolar correction in determining the electron concentration. The compensation balance in this material is controlled by the EL2 midgap defect, of which the fraction ionized depends on trace presence of CAs shallow acceptors, and of shallow donors, probably including SiGa . An increase of the latter towards the tail (top) end of one crystal led to a reduction of the EL2 ionized fraction, and a lowered resistivity—but one still within the conventional semi-insulating range. Carbon was measured from the strength of its local vibrational mode absorption, while near-infrared measurements showed that EL2 was present in a concentration ∼1016 cm−3, with relatively small variation across a wafer.

Effects of controlled As pressure annealing on deep levels of liquid-encapsulated Czochralski GaAs single crystals

Effects of controlled arsenic vapor pressure thermal treatments on deep levels of liquid-encapsulated Czochralski (LEC) GaAs, especially on the midgap donor EL2, are investigated. Undoped LEC n-type conductive and semi-insulating GaAs crystals are annealed in vacuum or under As vapor pressure. From depth profiles of EL2 concentration, it is confirmed that the thermal conversion on semi-insulating GaAs is caused by the reduction of EL2 concentration and the introduction of native acceptor (VAs). A three-step model for EL2 out-diffusion is proposed. The formation of EL2 occurs during the cooling process at a relatively low temperature of around 650 °C.

Redistribution of EL2 in undoped semi-insulating GaAs during annealing under arsenic overpressure

The variations of the EL2 concentration in undoped semi-insulating (SI) GaAs crystals with arsenic pressure, P AS2 , during high-temperature annealing have been investigated using infrared absorption measurements at 1.1 μm. It is found that the variations can be divided into two types within a range of P AS2 <2 atm, beyond which the EL2 concentration, not varying with P AS2 , approached an apparently saturated value which is correlated with the stoichiometry of as-grown crystals. An improvement on the uniformity of EL2 distributions is achieved after annealing. These results are in agreement with an analysis proposed in a previous paper.

Optical properties and origin of infrared light scattering centers in undoped semi-insulating GaAs crystals

The wavelength dependence and polarization characteristics of the infrared light scattered from an undoped GaAs crystal were investigated in the 90° angle infrared light scattering configuration. The scattering is Rayleigh scattering from scatterers which are always associated with the dislocations, and they are classified into three types,S, L A , andL G scatterers, according to their polarization characteristics. TheS, L A , andL G -scatterers are thought to be small As clusters, large As precipitates and large Ga precipitates, respectively.

Native Point Defects and Nonstoichiometry in GaAs (II) Mechanism of Formation and Degradation of Semiinsulating Properties of Undoped Gallium Arsenide Crystals

AbstractThe nature of the main electron trap, EL2, in undoped semiinsulating GaAs crystals have been studied both qualitatively and quantitatively. It has been shown that point defects which, most probably, are responsible for the EL2 level are antisite AsGa defects or (AsiVGs) complexes which are indistinguishable from the thermodynamic standpoint. The enthalpy and entropy of AsGa formation according to the reaction AsAs + VGa ⇄ AsGa + VAs are equal to 0.5 eV and −7k, respectively.

Quartz versus PBN&amp;#8212;The effect of crucible type on undoped LEC GaAS

Undoped semi-insulating GaAs crystals were grown in a low pressure LEC system using quartz and pyrolytic boron nitride (PBN) crucibles. Crystals grown in PBN crucibles are consistently semi-insulating from the seed end to the tail end. Crystals grown in quartz crucibles have lower Hall mobility. Other properties such as dislocation etch-pit of conductive material beginning from the seed end of the crystal, and have lower Hall mobility. Other properties such as dislocation etch-put density, implant profile, and thermal conversion characteristics of the two types of crystal are about the same.

Experimental Correlation between EPD and Electrical Properties in Undoped LEC As-Grown Semi-Insulating GaAs Crystals

Experimental Correlation between EPD and Electrical Properties in Undoped LEC As-Grown Semi-Insulating GaAs Crystals

Deep-center photoluminescence in undoped semi-insulating GaAs: 0.68 eV band due to the main deep donor

The 0.68 eV photoluminescence band present in undoped semi-insulating GaAs crystals has been studied with the change of temperature. It is shown that the 0.68 eV band is due to the radiative transition involving a main deep donor and the valence band. The origin of the donor is of an intrinsic origin and may involve an As antisite defect. It is found that the donor level does not change in energy with respect ot the valence band at T = 4–300 K. The donor level is found to be at 0.73 eV from the conduction band at T = 4 K.