Liquid-encapsulated Czochralski-grown GaAs Research Articles

Identification of individual and aligned microdefects in bulk vertical Bridgman- and liquid encapsulated Czochralski-grown GaAs

A systematic study of microdefects (precipitates) in undoped and Si-doped vertical Bridgman (VB)-grown and updoped liquid encapsulated Czochralski (LEC)-grown GaAs crystals was performed using infra-red (IR) and differential interference contrast (DIC) microscopy, diluted sirtl-like etchants used with light (DSL) etching, transmission electron microscopy (TEM) and X-ray topography. Apart from hexagonal As precipitates, which were reported in many publications, inclusions of crystalline As 2O 3 and As 2O 5 have been identified for the first time by selected-area diffraction (SAD) and from lattice spacing of Moiré patterns in undoped GaAs. The crystalline precipitates are often associated with voids. In Si-doped annealed GaAs, however, only voids have been found. The average size of precipitates in LEC material is about 100 nm, while in VB they can reach 600 nm. Most of the precipitates decorate dislocations (DPs), but some were also found in the dislocation-free matrix as single inclusions or linear arrays of precipitates (LAPs). The latter defect pattern has not yet been reported. We shall present a tentative formation mechanism of LAPs. From the TEM analysis it follows that the nature of these precipitates is the same as that of the DPs. It has been established that microdefects are surrounded by large (compared to the size of precipitates themselves) zones which have recombinative properties for photo-generated carriers, leading to the formation of hillocks during photo-etching before the defects are dissolved to form S-pits. The S-pits formed during DSL etching and jet etching in H 2SO 4:H 2O 2:H 2O solution used to prepare TEM specimens are elongated and run along the [1 1 ̄ 0] direction. From TEM it follows that the large microdefects in VB material have the form of a regular tetrahedron with the side walls bounded by {111} planes and the edges along the 〈110〉 directions.

High-Energy Ion-Implantation of a Moderately Deep Acceptor Hg Into Liquid Encapsulated Czochralski Grown GaAs : Formation of New Shallow Emission Bands

AbstractHg (mercury) in GaAs is known to be a moderately deep acceptor impurity, having a 52 meV activation energy. Optical properties of Hg acceptors in GaAs were systematically investigated as a function of Hg concentration, [Hg]. Samples were prepared by high-energy ion-implantation of Hg+ into GaAs grown by the liquid encapsulated Czochralski (LEC) method. Heat treatment was made by furnace annealing and rapid thermal annealing. Photoluminescence measurements at 2K revealed that the Hg-related so-called “g” line is formed in addition to the well-defined conduction band-to-Hg acceptor transition, (e, Hg). Additionally, three shallow emissions are formed for net hole concentrations INA-NDI greater than 2×1017cm−3 . This is the first demonstration that even Hg in GaAs makes multiple shallow emissions due to acceptor-acceptor pairs and LEC GaAs can be used for the investigations of these emissions.

Dislocation density-dependent photorefractive effect in (001)-cut GaAs

We present studies of the photorefractive effect in nonphotorefractive orientations of liquid-encapsulated Czochralski-grown GaAs crystals. Picosecond diffraction experiments conducted in different samples show that a forbidden photorefractive signal correlates well with dislocation density, which points out that the effect arises from strain fields and growth defects.

Observation of luminescence from the EL2 metastable state in liquid-encapsulated Czochralski-grown GaAs under hydrostatic pressure.

We have observed luminescence due to excitons bound to EL2 in its metastable state in both semi-insulating and n-type liquid-encapsulated Czochralski-grown GaAs. This luminescence appears at pressures, generated in a sapphire anvil cell, exceeding 3 kbar. An unexpected, drastic Fermi-level change quenches this luminescence at 10 kbar with commensurate changes in the deep EL2 luminescence.

Control of EL2 concentration and dislocation density in liquid encapsulated Czochralski-grown GaAs using laser processing

The authors report the first study of laser processing of GaAs under stoichiometry control. A series of experiments was performed to investigate the influence of laser processing on the electronic and structural properties of liquid encapsulated Czochralski-grown GaAs. A thermal treatment was used to convert samples from semi-insulating to conducting behaviour by severely reducing the EL2 concentration in the material. These samples were subjected to both laser processing and standard thermal annealing under stoichiometry control. Laser-assisted processing resulted in much stronger EL2 recovery rates than for standard thermal annealing and the tendency was more pronounced for arsenic-rich stoichiometry than with gallium-rich stoichiometry. Indeed, selected laser processing conditions were found to yield significantly higher sample resistivities than those of the starting material. Etch pit densities were studied as a function of the laser processing conditions and were found to be a minimum for near-stoichiometric conditions. A simple defect model is proposed and used to explain these experimental results.

Microstructure changes after annealing of undoped and Cr-doped liquid-encapsulated Czochralski-grown GaAs

Microinhomogeneities in semi-insulating, liquid-encapsulated Czochralski (LEC) GaAs crystals were investigated by selective photoetching and spatially resolved photoluminescence at 1.8 K. The interaction of point defects with dislocations was studied by annealing the samples in a metalorganic chemical vapor deposition reactor under AsH3 pressure. The changes in microstructure, induced by the anneal process, were investigated for both undoped and Cr-doped crystals. A weakening of the photoetching contrast after annealing was observed for both types of crystals, with no essential difference between undoped and Cr-doped ones. The overall near-band-gap photoluminescence (PL) intensities were found to have increased after annealing. This increase amounted to about one order of magnitude for the Cr-doped crystals, and two orders of magnitude for the undoped ones. In addition, for the undoped samples the contrast in PL intensity between dislocated areas and the undisturbed crystal matrix was increased by the anneal process. Analysis of the local PL spectra taken at different regions on these samples showed a strong enhancement of acceptor-bound exciton (A0,X) transitions relative to donor-bound exciton (D0,X) transitions, especially at dislocated areas. Furthermore, a new emission for LEC GaAs at 1.5116±0.0003 eV was also found to be strongly enhanced at these regions. The PL results are explained by the disappearance of deep, nonradiative, excess As-related PL killer centers during annealing. The disappearance is proposed to proceed via diffusion of interstitial As atoms and the formation of As precipitates, for which dislocations act as nucleation centers. Additional deep level photoluminescence spectra of the 0.65 and 0.80-eV emission bands are shown to be in accordance with the proposed model.

High spatial resolution photoluminescence studies of dislocations in Si-doped, liquid-encapsulated Czochralski GaAs

High spatial resolution photoluminescence experiments were performed on Si-doped, liquid-encapsulated Czochralski-grown GaAs. Typical defects investigated were grown-in dislocations surrounded by impurity atmospheres (G) and grown-in dislocations moved by thermal stresses during the post-growth cooling period leaving behind a glide trace (GS). Diluted Sirtl-like etching with the use of light was performed to localize the defects, which showed up as hillocks with a small summit at the dislocation outcrop. The luminescence experiments were carried out in a helium bath cryostat at 4.2 K using a newly developed focusing and translation mechanism, mounted inside the cryostat. A spatial resolution of ≊1 μm could thus be obtained. Grown-in dislocations, starting and ending points of GS dislocations, as well as their glide traces were all found to yield lower luminescence intensities than the crystal matrix. Moreover, a relative enhancement of the (D0,h) transitions with SiGa as a donor compared to the (e,A0) transitions with SiAs as an acceptor was observed at isolated grown-in dislocations and starting points of GS dislocations. Glide traces and ending points of GS dislocations, however, at which the photoluminescence intensity was also significantly lower than at the crystal matrix, did not show this enhancement. The results are explained on the base of a relatively high concentration of nonradiative deep centers at all of the above-mentioned defects, and a high concentration of shallow donors at grown-in dislocations and starting points of GS dislocations. The distinction found between different defects with regard to the impurity incorporation is explained in terms of the thermal history of the crystal.

Evidence of native gallium antisite defects in semi-insulating liquid-encapsulated Czochralski-grown GaAs

We report the observation by photoluminescence of the gallium antisite double acceptor in As-rich GaAs grown by the liquid-encapsulated Czochralski technique. In the semi-insulating material both arsenic antisite and gallium antisite defects are present simultaneously which must be taken into account for any electrical compensation model.

EL2 and gallium antisite defects in GaAs: Si

Photoluminescence (PL), Fourier-transform infrared (FTIR), and deep-level transient spectroscopy (DLTS) measurements have been made on various samples of silicon-doped liquid-encapsulated Czochralski-grown GaAs. All the samples show prominent PL peaks at 1.443 and 1.325 eV together with their longitudinal optic (LO) phonon peaks. The PL peak at 1.443 eV has been reported in the literature as being due to either GaAs or a boron-related defect. The FTIR results show the presence of BGa at 540.3 and 517.0 cm−1 and SiGa at 383.6 cm−1. We have observed that there is no correlation between the PL peak at 1.443 eV and BGa. Thus, we believe that this PL peak is related to the GaAs antisite defect. The presence of EL2 in the samples has been measured using DLTS. We have found that the intensity of the PL peak at 1.443 eV varies inversely with that of the EL2 peak. This relationship indirectly confirms that the 1.443 eV peak is due to the gallium antisite defect. The PL peak at 1.325 eV is significantly different from those reported in the literature for GaAs:Si. Measurements have also been made on samples of GaAs:Si annealed under different arsenic overpressures.

On the nature of the silicon activation efficiency in liquid-encapsulated Czochralski-grown GaAs by photoluminescence

The shallow defect evolution in Si-implanted and -annealed liquid-encapsulated Czochralski-grown GaAs is investigated by photoluminescence experiments. Three major emission lines are found. The first emission line located at 1.492 eV corresponds to the SiGa-CAs radiative recombination. The remaining two lines located at 1.44 and 1.40 eV are shown to correspond to GaI-SiAs and VAs-SiAs radiative recombinations, respectively. The effects of these three shallow centers on the silicon activation efficiency are discussed with respect to different annealing and implantation conditions.

A study of the distribution of hydrogen and strain in proton-bombarded liquid-encapsulated Czochralski-grown GaAs by double-crystal x-ray diffraction and secondary ion mass spectrometry

Proton bombardment is an effective method of providing electrical and optical isolation for devices in GaAs integrated circuits. In this study, n-type LEC GaAs has been implanted with protons at doses of 10 15 and 10 16 ions cm −2 at an energy of 190 keV. Double-crystal X-ray diffractometry of the as-implanted material reveals complex rocking curves featuring high intensity damage-related peaks. These have been analysed by comparison with simulated rocking curves which were calculated using the dynamic theory of X-ray diffraction with assumed strain profiles. This simulation-matching technique has enabled the determination of the strain distributions for the two doses studied. For the 10 15 cm −2 dose the peak strain is 3.8 × 10 −4 and for the 10 16 cm −2 dose it is 2.2 × 10 −3 with the peak strain for both samples occurring at a depth of 1.6 μm. Secondary ion mass spectrometry (SIMS) profiles confirm that the peak in the hydrogen concentration occurs close to the peak in strain. A further feature of the strain profiles in the as-implanted sample is that a close match with the experimental data is only obtained when a highly strained layer close to the surface is put into the simulation. Although this was unexpected, it was possible to substantiate its existence by etching studies. Subsequent annealing of the damage over the temperature range 150–500 °C (less than 200 °C in air, and more than 200 °C in arsine) has enabled modifications to the strain profile as a function of annealing temperature to be determined. There is no change at 150 °C but above this temperature the level of strain reduces as the annealing temperature increases and the structural damage recovers between 400 and 500 °C which approximates to the optimum annealing temperature required to achieve good isolation properties. SIMS profiles of the annealed samples show that hydrogen is not mobile at 150 °C but above 150 °C it moves down the strain gradient towards the surface. The data are discussed in the light of the currently perceived model of radiation damage annealing in GaAs, i.e. there are two recovery stages at about 200 °C and 400 °C.

Photorefractive gain in GaAs under a dc electric field

We report the first observation of a photorefractive gain coefficient as high as 2.6 cm−1 in the undoped liquid-encapsulated Czochralski-grown GaAs crystals at 1.06 μm under a dc electric field of 13 kV/cm without using the moving grating technique. The absorption coefficient of the crystals used is 1.3 cm−1, showing that a net gain has been achieved. This measured gain coefficient is close to the predicted theoretical value.

Influence of striations in GaAs on the activated profiles of implanted silicon

Liquid-encapsulated Czochralski-grown GaAs is known to show radial striations. In GaAs:In they can be seen by x-ray topography via fluctuations of the In concentration which cause variations in the lattice constant. Using a high spatial resolution capacitance-voltage technique we investigated In-doped as well as undoped GaAs. It is demonstrated that both materials exhibit striations in the activation behavior of implanted silicon and hence in the threshold voltage of ion-implanted metal-semiconductor field-effect transistors. In the cases investigated here the threshold voltage varied by up to ±12 mV from peak to peak. To our knowledge this is the first time that the effect of striations on the activation behavior of implanted Si is demonstrated in both kinds of substrate material.

Laser-induced degradation of GaAs photoluminescence

We report experimental studies of laser-induced degradation of surface photoluminescence efficiency in n-GaAs, p-GaAs, semi-insulating Cr-doped, and undoped liquid-encapsulated Czochralski-grown GaAs for excitation intensities ranging from ∼0.1 to ∼20 kW/cm2. The data suggest a contribution from optically induced defects in the bulk material. The time dependence of photoluminescence efficiency was fitted to a simple power-law expression. A previously unreported fast decay of photoluminescence, occurring in 1–2 s immediately following the cleaving of a fresh surface, was observed in p- and n-type samples.

Anomalous reduction of lattice parameter by residual impurity boron in undoped Czochralski-grown GaAs

An anomalous reduction of lattice parameter is observed for undoped semi-insulating liquid encapsulated Czochralski-grown GaAs by precision lattice parameter measurements with the use of the bond method. The anomalous reduction is though to be due to the incorporation of residual boron from the B2O3 encapsulant. The reduction rate is about 2 times larger than the predicted rate for the boron concentration derived from Vegard’s law. Possible defect models for this phenomenon are discussed.

Deep levels in semi-insulating liquid encapsulated Czochralski-grown GaAs

Photo-induced transient spectroscopy (PITS) was performed on variously heat-treated samples of semi-insulating liquid encapsulated Czochralski-grown GaAs. Seven deep levels at 0.57, 0.52,0.42, 0.36, 0.27, 0.22 and 0.18 eV were observed. These levels can be identified with levels seen using other deep-level techniques and a variety of crystal growth and sample preparation techniques. The levels at 0.52, 0.42 and 0.36 eV can be annealed out by heat treatment. These levels therefore seem to be associated with structural defects rather than impurities.

Image processing by four-wave mixing in photorefractive GaAs

Three image processing experiments were performed by degenerate four-wave mixing in photorefractive GaAs. The experiments were imaging by phase conjugation, edge enhancement, and autocorrelation. The results show that undoped, semi-insulating, liquid-encapsulated Czochralski-grown GaAs crystals can be used as effective optical processing media despite their small electro-optic coefficient.

High resolution scanning photoluminescence characterization of semi-insulating GaAs using a laser scanning microscope

Spatially resolved photoluminescence properties of semi-insulating, liquid encapsulated Czochralski-grown GaAs substrates are analyzed with a laser scanning microscope. The improved resolution of the laser scanning microscope results in the observation of single dislocations within the subgrain boundaries of the polyganized dislocation cell network for the first time by photoluminescence. Both the cell structure and the Cottrell cloud are clearly resolved.

Dislocation effects on carrier concentration for molecular-beam-epitaxially grown GaAs

Dislocation effects on carrier concentration for Si-doped and Si-implanted molecular-beam-epitaxially (MBE) grown GaAs layers are investigated by small area (40×40 μm2) Hall measurements. It is found that there is no correlation of carrier concentration with dislocation density and distance from a dislocation to a Hall device. The photoluminescence 1.51-eV bound exiton peak and the 1.49-eV carbon acceptor peak intensity variations are small across a wafer. Moreover, there are no variations in high-resolution photoluminescence intensity and in electron-beam-induced current image between positions close to and far from a dislocation. These results indicate that carrier concentration is not affected by dislocations in MBE-grown epilayers because of the absence of a denuded zone around the dislocation. Acceptors and/or point defects are depleted in liquid-encapsulated Czochralski-grown GaAs, whereas this does not take place during the MBE growth processes.

Identification of a vanadium-related level in liquid encapsulated Czochralski-grown GaAs

We present the first positive identification of a vanadium-related electron trap in V-doped GaAs crystals grown by the liquid encapsulated Czochralski technique in pyrolytic boron nitride crucibles. Detailed deep level transient spectroscopy and capacitance transient analysis yielded a trap energy of 0.15±0.01 eV below the conduction band and an electron capture cross section of about 2×10−14 cm2. Optical absorption and mobility data show that this level corresponds to the ionized acceptor state V2+(3d3) of substitutional vanadium. No midgap levels other than EL2 could be detected in these V-doped crystals showing that doping with vanadium plays no direct role in the compensation process in semi-insulating GaAs crystals.