Growth and characterization of GaAs crystals produced by the VCz method without boric oxide encapsulation
Introduction
Since the requirements for GaAs substrates used for epitaxial circuits instead of ion-implanted ones are continuously increasing [1], the focus has changed towards improvements of the wafer surface quality. One of the most important targets is the prevention of harmful second-phase particles, so-called crystal-originated pits (COPs), affecting the surface preparation and thin film deposition process. In standard as-grown crystals obtained by the liquid encapsulation Czochralski (LEC) and vertical gradient freeze (VGF) techniques, this problem is related to the non-stoichiometry due to the growth from slightly arsenic-rich melts [2], leading to formation of As precipitates [3] and probably, inclusions. As it is well known for a long time [4] that even after post-growth annealing, there are still related residual defects forming crater-shaped pits during polishing.
Another problem related to the off-stoichiometry is the enhanced content of intrinsic point defects like As interstitials, vacancies of both types and As antisites. Especially the last one, identified as the deep-level donor EL2 (AsGa) in low concentrations of 1016 cm−3, plays an important role for electrical properties. It is favourable to ensure stable semi-insulating (SI) behaviour [1], but disadvantageous for application of such crystals that are used in high-sensitivity radiation detectors [5] due to their substantial contribution to the carrier recombination and hence, to their lifetime reduction. In the present work, the efforts were directed on the growth of near-stoichiometric crystals with reduced excess of arsenic. For this, the growth from in situ controlled Ga-rich melts by the VCz method [1], [2] without boric oxide encapsulant was used.
In principle, the attempts to grow GaAs by the Czochralski method without boric oxide encapsulant are very seldom, but not new, and go back to the 1950s [6] and 1960s [7]. During the 1980s, the hot-wall Czochralski (HWC) technique with controlled As atmosphere was developed [8], [9], but receded somewhat into the background. Recently, we again took up this idea [10], [11], however, based on our long-term experiences on VCz developments [12], and on a markedly improved structural material and construction level.
Section snippets
Experimental procedure
Undoped GaAs crystals with diameters between 2 and 3 in have been grown from different Ga-rich melt compositions by the VCz method without boric oxide encapsulant, described in detail elsewhere [9], [10]. Argon or nitrogen with a pressure of 0.5 MPa was used as process gas. The mole fraction of the melt yL was controlled in situ in the range by the partial arsenic pressure between 0.02 and 2.1 MPa adjusted via the temperature of the As source from 540 to 650 °C. Experimentally, the
Crystal phenotype
Fig. 1 shows images of two VCz single crystals grown from Ga-rich melts without boric oxide encapsulation. It is worth mentioning that the surfaces are mirror-like and free of any symptoms of dissociation as it is usual for LEC crystals [13]. Markedly enlarged {1 1 1} facets seen in Fig. 1a are caused by the low radial temperature gradient evolved from the absence of the B2O3 layer. In consequence, an enhanced twinning probability should be feared. Using highly purified and freshly synthesized
Conclusions
The growth feasibility of GaAs VCz crystals without boric oxide encapsulant from in situ controlled Ga-rich melts was reported. The following qualities have been obtained in detail:
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twin-free single crystals,
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near-stoichiometric crystal composition with markedly reduced arsenic precipitates and without Ga inclusions,
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low and radial homogeneously distributed dislocation density ⩽104 cm−2 without rim effects,
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reduced boron, as low as 1015 cm−3,
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no oxygen defects and related complexes,
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controlled carbon
Acknowledgements
The authors are grateful to M. Czupalla, Dr. Ch. Frank-Rotsch and Dr. M. Neubert for helpful contributions to the growth developments, B. Lux, Th. Wurche and M. Imming for crystal preparation, M. Naumann for LST, U. Juda and K. Banse for EPD analysis, and Dr. K. Irmscher (all from IKZ, Berlin) for Hall measurements. They are indebted to Prof. W. Ulrici from PDI, Berlin, for the LVM analysis. The work was supported by the German Research Association (DFG) under Contract nos. RU 505/10-1, RU
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