Using H2O as a transport agent, epitaxial GaAs layers were grown by the close-spaeed vapor transport technique (CSVT) on (100) heavily Si-doped GaAs substrates. Three kinds of GaAs sources were used for the deposition: (100) GaAs, heavily doped with Te or Si, and undoped semi-insulating (SI) (100) GaAs. The growth rates obtained with SI and Te-doped GaAs are quite similar and show a clear tendency to be superior to the growth rates measured for Si-doped GaAs sources. Uncompensated charge carrier density (ND – NA) profiles have been measured electrochemically for the layers grown with the three kinds of sources. When Te-doped GaAs is used, (ND – NA) obtained for the epitaxy is the same as that of the source, implying a complete transfer of the Te impurity. (ND – NA) values varying from 1016 to 1018 cm−3 are obtained from SI GaAs sources, depending upon the thickness of the epitaxial layer. (ND – NA) < 1015 cm−3 are measured for layers grown from Si-doped GaAs sources. In this case, layers thicker than 10 μm cannot be mesured electrochemically because of their excessively high resistance. The small (ND – NA) values obtained in that case are explained by the reaction of Si contained in the source with the transport agent (H2O), resulting in the formation, at the Si-doped GaAs surface, of a passivating SiOx layer revealed by Auger spectroscopy. This passivating layer also explains the smaller growth rates measured with these sources. p–n Junctions have been prepared by Zn diffusion in CSVT layers grown from SI GaAs sources. Their I–V characteristics show good rectification behavior, indicating that the CSVT layers could be used for photovoltaic purposes.
Epitaxial layers of GaAs on (100) GaAs substrates can be grown by close-spaced vapor transport using water vapor as the transporting agent. The parameters for the transport are Ti = 755°C. AT' = 45°C. and 6 = 0.03 cm (where Ti is the temperature of the graphite heating the substrate; AT', the temperature difference between the graphite heating the source and the one heating the substrate; and 6. the thickness of the spacer separating the GaAs source and the substrate). Mirrorlike epitaxial layers of GaAs are obtained with these parameters when water vapor, at a partial pressure of 4.58 Torr ( 1 Torr = 133.3 Pa), is introduced with H, at the beginning of the temperature rise of the reactor. The dimensions of the epitaxial layer are only limited by the size of the reactor. Using the same growth conditions. it is not possible to obtain mirrorlike films of GaAs on (100) Ge substrates. Instead, the layers are dull grey (sample no. I ) . It is howcder not a polycrystalline deposition since the pole figures, obtained by X-ray diffraction. reveal only four crystallographic orientations: {loo} the main one. {221} the secondary one, and {021} + {I 12) two minor contributions. Mirrorlike films of GaAs on (100) Ge substrates of less than 1 cni' have been obtained with T: = 775OC. AT' = 25°C. and 6 = 0.03 cm. With these conditions, the growth rate is 0.25 + 0.08 k m m i n I . The time evolution of T: and AT', froni room temperature up to the equilibrium temperature also influences the surface morphology of GaAs films on Ge while this was not the case for GaAs films on GaAs substrates. When the Ge substrate is larger than I cm'. the centre of the film becomes textured but the edges reniain mirrorlike (sample no. 2). Pole figures obtained for the center and the edges of sample no. 2 are similar. They are characterized by one large diffraction due to the (100) orientation. A few random crystallographic orientations and sometimes the {221} orientation, however, bearly emerge froni the background of these pole figures. Also transmission electron microscopy does not reveal any major difference between the center and the edges of sample no. 2. The density of threading dislocations is the sanie for both regions, varying from 10' c m ', close (2-3 k m ) to the interface, to lo7 c m ' in the thickness of the film. No misfit dislocations were observed.Antiphase boundaries are present in both regions as well. The only difference between the centre and the edges of sample no. 2 involves microtwin bundles: in the center region, there are two microtwin bundles per micrometre of interface, extending up to 6 km in the GaAs film while on the edges, there is one bundle per micrometre with an extension of only one micrometre into the epitaxial layer. Mirrorlike GaAs films can be obtained on (100) Ge substrates of at least 1 in ( 1 in = 2.5 cni) in diameter by increasing 6 to 0.2 cm and by injecting water vapor in the reactor only when Ti reached 650°C: the other deposition parameters reniain the sanie as for sample no. 2. In these conditions, th...
The close-spaced vapor transport (CSVT) technique is used to grow GaAs epitaxial layers from various n- or p-type doped GaAs sources. The transport agent is H2O with PH2O = 4.58 Torr. n-type layers can be grown with Te- or Ge-doped GaAs sources. The transport coefficients of both dopants (ratio of the electrically active dopant concentration in the layer to the electrically active dopant concentration in the source) is 100% for Te or Ge, in the substrate temperature range comprised between 750 and 850 °C. p-type layers are obtained with Zn-doped GaAs sources. The transport coefficient of Zn is about 1% and is also independent of the substrate temperature. The transport coefficients and their independence on temperature are in agreement with a mass-transport controlled model based on the hypothesis that the transport reactions of GaAs and the doping impurities are in equilibrium at the source and substrate temperatures. Si-doped GaAs cannot be used as a source to obtain conductive n-type layers. When undoped semi-insulating (SI)-GaAs wafers are used as sources in CSVT, n-type layers are obtained. They are characterized by ND−NA=9×1015–3×1016 cm−3 and μ300K=3000–4000 cm2 V−1 s−1, independent of the temperature, in the temperature range investigated. Glow discharge mass spectroscopy analyses performed on a source and on a layer indicate that C, O, Si, and S are the major residual impurities in the GaAs layer. All these impurities have their origin in the technique (reactor, transport agent). Ge is also present in the layers, as indicated by photoluminescence. It is a minor impurity. Its origin is probably the SI-GaAs source.
GaAs epitaxial layers have been grown by close-spaced vapor transport (CSVT) from various p-type Zn doped GaAs sources, using H20 as transporting agent. The doping impurity concentrations in the sources were in the 10 TM, 10 I8, 1019, or 1020 cm -~ ranges. Heavily compensated n-type epitaxial layers were obtained with sources characterized by an acceptor 16 18 concentration in the i0 -I0 range, while p-type layers were obtained with sources having Zn concentrations equal or greater than i0 TM cm -3. About 1% of the Zn present in the source is found to be electrically active in the epitaxial layer. With the help of a diffusion control model based only on thermodynamic analysis, it is possible to demonstrate that Zn is transported as ZnO. Using the same model, it is also shown that Te, a n-type doping impurity characterized in CSVT by a transfer coefficient of unity, is transported as Te2 under the same growth conditions.Close-spaced vapor transport (CSVT) is a special kind of chemical vapor deposition (CVD) technique developed about 25 years ago almost simultaneously by Sirtl and a RCA team (1-5). Its particularity lies in the displacement of the thermal equilibrium of the transport reaction resulting from different source and substrate temperatures. This provides the driving force of the transport. During the deposition, the source and substrate are close together in the reactor. The transporting agent, such as H20 used in the transport of GaAs, is introduced into the reactor with the help of a carrier gas such as H2. All the problems relative to the storage and use of highly toxic reagents necessary in conventional CVD of GaAs are therefore avoided in CSVT.It has been demonstrated that GaAs wafers can be used as sources in CSVT to grow epitaxial layers of GaAs on GaAs substrates (6-11) and also on Ge substrates (1-4). However, in order to produce by CSVT solid-state_ devices like solar cells, it is necessary to control the electrical properties of the grown layers. An attempt toward this goal is the use of doped GaAs wafers as the source of reagents. In this case, As2 (or As4) and Ga20 obtained by the reaction of GaAs with H20 (the transporting agent) are provided along with the doping impurity to be incorporated in the growing layer. It has been shown (12) that n-type Te-doped GaAs sources can be used to grow n-type doped layers with a Te transfer coefficient equal to unity. By comparison (11), layers grown from undoped semi-insulating GaAs sources are also n-type but with a background doping density in the low 1016 cm -3. The behavior of Si-doped GaAs sources has also been investigated (12). In that case, the oxidation of Si by water impedes its transfer. SiOx accumulates at the surface of the source where it can be detected by Auger spectroscopy.It is the purpose of this work to demonstrate that p-type GaAs layers can be obtained by CSVT using Zn doped GaAs sources and H20 as transport agent. However, it is shown that only -1% of the Zn present in the source is found to be electrically active in the epitaxial...
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