Ken Yamashita

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.

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.

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...

Crystallization of amorphous Si on a glass substrate through nucleation by Si+ ion implantation

Crystallization of amorphous Si films on a glass substrate by Si+ implantation (acceleration energy: 180 keV, beam current density: 10 μA/cm, ion dose: 1×1017 ions/cm2) was performed without external heating of the substrate. Transmission electron microscopy images of the crystallized specimens lead to the following conclusions: (1) crystallization was achieved through bulk nucleation by Si+ implantation, which is a low temperature and rapid process compared with the ordinary thermal process, (2) the crystallization is strongly related to the ion‐solid interaction, not due to ‘‘pure’’ thermal annealing by ion beam heating.

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.

Dependence of source gas mole ratio on crystalline quality of ZnSe layers grown on (100) ZnSe substrates by atmospheric pressure metalorganic vapor phase epitaxy

Abstract Single crystalline layers of undoped ZnSe have been grown on (100) ZnSe substrates at 250°C by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) using dimethylzinc (DMZ) and hydrogen selenide (H 2 Se). The surface morphologies, observed by a Nomarski phase contrast interference microscope, depend on the growth conditions, particularly the source gas mole ratio η M ( η M =[Se]/[Zn]). The surface morphologies of homoepilayers grown with η M of more than 15 are very rough like polycrystals. However, these layers have the best crystalline quality as determined by a double crystal X-ray diffraction spectrometer (the full width at half maximum (FWHM) of (400) diffraction is 33 arc sec). The surface morphologies of layers grown at values of η M between 3 and 15 are excellent as well as those of heteroepilayers (GaAs substrates are used). On the other hand, the crystalline quality becomes worse by reducing η M . The photoluminescence (PL) spectra at 4.2 K exhibit strong and well-resolved band-edge emissions from the donor bound exciton (hereafter called I x ) and the free exciton (hereafter called E x ) only for the low mole ratio (η M x is 0.8 meV which is sharper than that of heteroepilayers. However, the intensity of emissions from deep levels (deep emissions) of homoepilayers measured at 77 K is several times stronger than that of heteroepilayers. In the region of high mole ratio (η M >10), the PL spectra at 4.2 K show two broad lines: I x and the unknown I v (probably donor bound exciton). And the intensity of I x reduces two orders by decreasing the mole ratio from 20 to 3. This indicates that the concentration of donor decreases for the Se-rich condition. From the investigation of PL measurements, the origin of deep emissions is considered to be caused by complexes of Zn vacancies and impurities from the source gases. The dependences of η M on the quality of homoepilayers observed in this work are quite different from those in heteroepilayers reported previously.