Tokuo Yodo

Growth of high-quality ZnSe by MOVPE on (100) ZnSe substrate

Single crystalline layers of undoped ZnSe have been on (100) ZnSe substrates by atmospheric pressure MOVPE using dimethylzinc (DMZ) and hydrogen selenide (H2Se). The layers are typically 6 μm thick and the surface morphologies, measured by Nomarski phase contrast interference microscopy, appear superior to those of ZnSe layers grown on GaAs subrates. The photoluminescence (PL) spectra at 4.2 K exhibit strong band edge PL peak dominated by donor bound exciton (DBE) and free exciton (FE) emissions. The full width at half maximum (FWHM) of DBE is 0.8 meV and sharper than that of layers grown on GaAs substrates. However, the intensity of the self-activated (SA) PL of the homoepilayer measured at 77 K is several times more intense than that of heteroepilayers. From the investigations on the gas source molar ratio, it is inferred that the reason for this is that Zn vacancies diffuse from the substrate into the layer even at a growth temperature as low as 250°C. The characterization of the crystallographic properties by double X-ray diffraction rocking curves shows that the crystalline quality of the homoepilayers is drastically improved by heat treatment (300–650°C) of the ZnSe substrate under hydrogen atmosphere in the reactor before growth. The FWHM of the (400) diffraction of the homoepitaxial layer grown under the optimum conditions is 77 arc sec and this value shows that the ZnSe layer has the highest crystalline quality which has ever been obtained by any growth technique.

Li‐doped ZnSe epitaxial layers by ion implantation

Li+ ions were implanted into undoped high crystalline quality ZnSe epilayers grown by metalorganic vapor phase epitaxy. Thermal annealing after ion implantation was performed between 451 and 757u2009°C. The total photoluminescence intensity (measured at 77 K) of implanted layers before annealing was two orders of magnitude less than that of an as‐grown one because of the Li+ ion damage. As the annealing temperature was increased, however, the damage was reduced and the photoluminescence intensities of two main peaks (band‐edge emission and strong free to acceptor transition emission) became intense without generating deep centers. At 4.2 K, the only emission observed was associated with acceptor bound exciton and donor acceptor pair recombination in the near‐band‐edge region.

Growth of ZnSe single crystals by iodine transport

Abstract Large ZnSe single crystals (14x14x20 mm3) have successfully been grown from the vapor phase by iodine transport. Substrates with large area (10x10 mm2) for epitaxial growth were prepared by cutting the crystal. It is important to control the temperature fluctuation and seed direction for rapid growth. As an etchant for the measurement of dislocation density, the NaOH solution on (111)B is better than the Br-CH3OH solution on (111)A. The etch-pit density is (2–7)x104/cm2. The iodine concentration in the crystal is about 200 ppm. The resistivity of the ZnSe crystal annealed in molten Zn is 0.03 Ω cm.

Epitaxial Growth of High Quality ZnSe on GaAs Substrate by Atmospheric Pressure MOVPE Using Dimethylzinc and Hydrogen Selenide

Single crystalline layers of undoped ZnSe have been grown on GaAs substrates by atmospheric pressure MOVPE using dimethylzinc (DMZ) and hydrogen selenide (H2Se). Premature reactions typically encountered with this source combination can be eliminated completely, even at atmospheric pressure, by controlling the flow velocity of each source gas and the source gas mole ratio. For the first time, an excellent mirror surface morphology, as revealed by Nomarski interference microscopy, was obtained even for 8.2 µm thick epilayers grown at 300°C.

Thermal stability of ZnSe epilayer grown by MOVPE

The thermal stability of ZnSe heteroepitaxial layers grown at 250° C by the atmospheric pressure MOVPE is much better than that of heteroepitaxial layers grown at 300° C by conventional reduced pressure (0.1 Torr) MOVPE. The thermal stability of epitaxial layers is related to the growth conditions, which would determine the degree of stoichiometry in the epitaxial layers. However, heat treatment higher than 600° C in hydrogen ambient easily generates a high concentration of deep centers and promotes incorporation of impurities from the surface by some chemical reaction of ZnSe with hydrogen irrespectively of the degree of stoichiometry. On the other hand, an introduction of Zn vapor in H2 has the effect of suppressing the generation of deep centers. It is strongly suggested that Zn vacancy is directly related to the origin of deep centers. A reduction of its concentration under controlled atmosphere during high-temperature treatment is required to keep the high crystalline quality. Also the heat treatment in nitrogen ambient does not generate the deep centers even at 657° C. Consequently, it is found that ZnSe epilayer grown at 250° C by the atmospheric pressure MOVPE is thermally stable at least up to about 700° C.

Growth of CdS by atmospheric pressure metalorganic vapor‐phase epitaxy at low temperature

Single‐crystalline epilayers (epitaxial layers) of undoped, phosphorus‐, and iodine‐doped CdS have been grown at 250–400u2009°C on (100)‐oriented GaAs substrates by atmospheric pressure metalorganic vapor‐phase epitaxy using dimethylcadmium and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination, as well as in the case of ZnSe epitaxial growth using dimethylzinc and hydrogen selenide, can be eliminated completely even at atmospheric pressure by controlling the respective flow velocities of the carrier gases, including the source gases, independently. The crystallographic structure of CdS epilayers is largely related to the VI/II source gas mole ratio, and changes from hexagonal into cubic (100) structure with decreasing VI/II ratio. From the x‐ray‐diffraction and photoluminescence measurements, an epilayer grown at 350u2009°C with a VI/II ratio of 2 has excellent crystalline quality with complete single‐cubic (100)‐type epitaxial structure. Also in corre...

High‐quality epitaxial growth of ZnSe on (100) ZnSe by atmospheric pressure metalorganic vapor phase epitaxy

Single crystalline layers of undoped ZnSe have been grown on ZnSe substrates at 250u2009°C by atmospheric pressure metal organic vapor‐phase epitaxy using dimethylzinc and hydrogen selenide (H2 Se). The substrates are ZnSe wafers with (100) orientation cut from bulk crystals grown by iodine vapor transport. A homoepitaxial layer (homoepilayer) of ZnSe with excellent crystallographic properties, as revealed by double crystal x‐ray diffraction, was obtained. The full width at half maximum (FWHM) of the (400) diffraction pattern was 38 arcsec, which was superior to the crystallographic property (the FWHM was 250 arcsec) of the heteroepitaxial layer (heteroepilayer; a GaAs substrate was used). However, the intensity of deep emissions near 560 nm (PL at 77 K) is several times stronger than that of the heteroepilayer. The origin of deep emissions is not supposed to be due to iodine or copper diffused from the ZnSe substrate but to complexes between Zn vacancies and impurities in the layer.

Crystallographic properties of ZnSe grown by sublimation method

Abstract The crystallographic properties of ZnSe grown by the sublimation method were investigated. The (111) B face of the seed crystals was suitable for crystal growth. The crystal morphology was the hexagonal prism and they were constructed by {110} faces only. These properties were compared with the crystal grown by the iodine transport method. The seed direction and crystal morphology were different between the two methods. Particularly, the twin density of the crystal by the sublimation method was 5–6 mm -1 , and was higher than the crystal by the iodine transport method. This high value relates to the hexagonal prism morphology and the change of the surface energy when the twin was generated.

Epitaxial growth of ZnS grown at low temperatures by atmospheric pressure metalorganic vapor phase epitaxy

Single crystalline epitaxial layers of undoped ZnS have been grown at 250–300 °C on (100)‐oriented GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy using dimethylzinc (DMZ) and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination can be eliminated completely as well as in the case of ZnSe epitaxial growth using DMZ and hydrogen selenide, even at atmospheric pressure, by controlling the source gas velocity and the mole ratio. ZnS heteroepitaxial layers (heteroepilayers) with excellent mirrorlike surface morphologies, which were grown at 250 °C for the source gas mole (VI/II) ratio below 10, were obtained. The VI/II ratio dependence of the ZnS heteroepilayers is very different from that of ZnSe heteroepilayers, whose surface morphologies are mirrorlike at a VI/II ratio above 10. Also the surface morphologies of the ZnS heteroepilayers gradually begin to degrade as the growth temperature increases above 250 °C. Moreover, below 220 °C, the crystalline quality abruptly changes to polycrystal, as with bad hazy morphology, which is similar to that seen in ZnSe heteroepilayers grown below 210 °C. It is shown that it is necessary to grow ZnS layers epitaxially at least higher than 220u2009°C.Single crystalline epitaxial layers of undoped ZnS have been grown at 250–300 °C on (100)‐oriented GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy using dimethylzinc (DMZ) and hydrogen sulfide as source materials. The premature reaction typically encountered with this source combination can be eliminated completely as well as in the case of ZnSe epitaxial growth using DMZ and hydrogen selenide, even at atmospheric pressure, by controlling the source gas velocity and the mole ratio. ZnS heteroepitaxial layers (heteroepilayers) with excellent mirrorlike surface morphologies, which were grown at 250 °C for the source gas mole (VI/II) ratio below 10, were obtained. The VI/II ratio dependence of the ZnS heteroepilayers is very different from that of ZnSe heteroepilayers, whose surface morphologies are mirrorlike at a VI/II ratio above 10. Also the surface morphologies of the ZnS heteroepilayers gradually begin to degrade as the growth temperature increases above 250 °C. Moreover, below...

Na+ ion implanted ZnSe epitaxial layers grown by atmospheric pressure metalorganic vapor phase epitaxy

We have introduced Na impurities by ion implantation into undoped ZnSe heteroepitaxial layers grown at 250u2009°C by atmospheric pressure metalorganic vapor phase epitaxy. The implanted Na impurity forms not only an acceptor level but also a deep level. The formation of implanted Na acceptor levels begins to increase at relatively low annealing temperatures (less than 450u2009°C). Annealing between 451 and 557u2009°C is the optimum condition to form acceptor levels effectively while generating a low density of deep levels. Annealing of temperatures significantly higher than 550u2009°C does not form Na acceptors, but generates deep levels.