Kenei Ishino

Strong luminescence from dislocation-free GaN nanopillars

GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300nm, 0.5–1μm, and 3–4×108cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.

Growth of GaN films by hot wall epitaxy

GaN films were prepared by hot wall epitaxy on sapphire (0001) substrates from Ga and NH3 sources. Growth characteristics of the GaN films were investigated from reflection high energy electron diffraction (RHEED) and x‐ray diffraction measurements, and effects of initial layers on the film growth are discussed. High quality films with streak RHEED patterns were obtained when the films were grown on a GaN initial layer prepared by Ga predeposition and its nitridation on surface nitrided sapphire substrates.

Properties of CdS-ZnS superlattices prepared by hot wall epitaxy

Abstract CdS-ZnS superlattices (SLs) have for the first time been prepared on GaAs (100) substrates by hot wall epitaxy, using a flip-flop (growth-interruption) process. Single crystalline (cubic) patterns were observed in reflected high energy electron diffraction measurements for SLs consisting of thin layers of less than 30 A. Their SL structures were ascertained by the observation of satellites in the X-ray diffraction spectra. Electronic properties of the SLs have been studied through photoluminescence (PL) measurements in the temperature range of 10–300 K. Very strong PLs associated with the band gap of the SLs have been observed in the photon energy region higher than 2.5 eV (the energy gap of CdS, green), even though the lattice mismatch between CdS and ZnS is 7.5%. The peak photon energy was analyzed using the Kronig-Penney model. The results indicate that the SL is type I: Both electrons and holes are confined in the CdS layer which has the smaller energy gap.

Quantum-Cascade Structure in AlN/GaN System Assisted by Piezo-Electric Effect

AlN/GaN quantum-cascade structures which utilize piezo-electric fields to inject electrons into second conduction subbands are proposed. The (AlN)1/(GaN)n short-period superlattice has a large first-subband broadening owing to a large interaction of wave functions between quantum wells, and it is useful to obtain the population inversion between the first and second subbands. Effective electron injection into the second subband is made possible by inserting several atomic layers of AlN periodically into the (AlN)1/(GaN)n short-period superlattice.

Laser application of Pb1−xSrxS films prepared by hot wall epitaxy

Pb1-xSrxS films and Pb1-xSrxS/PbS double-heterostructure lasers were prepared using a hot wall epitaxy technique. Pb1-xSrxS films with energy band gaps up to 1.1 eV (x=0.15) were obtained. The band gap increased very rapidly with the SrS content as dEg/dx=7.5 eV (x<0.04), while the lattice constant increased as da/dx=0.034aPbS. Impurity dopings were performed and electrical properties were measured for the Pb1-xSrxS films. Films with large p- or n-type concentrations were obtained by doping with Tl or Bi impurities. Pb1-xSrxS/PbS double-hetero diode lasers with broad area and stripe contacts were fabricated. Laser operations were obtained up to 210 K pulsed for the broad-area laser and 245 K pulsed (174 K cw) for the stripe contact laser around 3 mu m.

PbCaTe Films and PbCaTe/PbTe Superlattices Prepared by Hot-Wall Epitaxy

PbCaTe films, and PbCaTe/PbTe and CaTe/PbTe superlattices were prepared by hot-wall epitaxy for application to mid- and far-infrared lasers of the 4 to 20 µm range. The PbCaTe ternary alloy shows good lattice matching with PbTe, and high carrier concentration films with a band gap wider than 650 meV were obtained by doping Bi and Tl as donor and acceptor impurities, respectively. PbCaTe/PbTe and CaTe/PbTe superlattices with very thin CaTe layers were also prepared. Optical transmission measurements were performed for the superlattices. Steplike interband absorption due to two the dimensional density of states of the superlattices was observed at room temperature. The absorption edge of the PbCaTe/PbTe superlattices agreed well with the theoretical values calculated by the envelope-function approximation.

Characteristics of Chlorine-Doped ZnSe Films and ZnSe–ZnS Superlattices Grown by Hot Wall Epitaxy

Chlorine-doped (Cl-doped) n-type ZnSe films were prepared on GaAs(100) substrates by hot wall epitaxy (HWE) using ZnCl2 as a doping source. The electron concentration could be controlled from 8.4×1014 cm-3 to 2.8×1019 cm-3 by varying the ZnCl2 temperature. ZnSe films with an electron concentration above 1019 cm-3, having a donor-bound excitonic photoluminescence emission (I2) without deep level emission were obtained for the first time. The activation energy of the chlorine donor was estimated to be 26.7 meV from the photon energy of I2. The existence of Cl in the films was confirmed by SIMS. Moreover, the Cl-doped ZnSe–ZnS superlattices with high electron concentration on the order of 1017 cm-3 and excitonic PL emissions associated with the free exciton, were obtained for the first time.

Characteristics of Nitrogen-Doped ZnTe Films and ZnTe-ZnSe Superlattices Grown by Hot Wall Epitaxy

Nitrogen-doped (N-doped) p-type ZnTe films have been prepared on GaAs(100) substrates by hot wall epitaxy (HWE) for the first time. To obtain high-quality films with high hole concentrations, optimum growth conditions such as the substrate temperature and the growth rate were studied by X-ray and Photoluminescence (PL) measurements. The hole concentration and Hall mobility were 1.1×1017 cm-3 and 52 cm2 V-1 s-1, respectively. The PL spectra of these films had a excitonic emission (I1), indicating high crystalline quality. The activation energy of the nitrogen acceptor was calculated, for the first time, to be 51 meV from the donor-acceptor (DA) emission energy. The existence of nitrogen in the films was confirmed by secondary ion mass spectroscopy (SIMS). The N-doped ZnTe-ZnSe SLs were also prepared and the hole concentration and Hall mobility were 2.3×1018 cm-3 and 36 cm2 V-1 s-1, respectively.

ZnS:Mn Electroluminescent Films Prepared by Hot Wall Technique

Manganese-doped zinc sulfide electroluminescent (EL) films were prepared for the first time by a hot wall technique with Mn planar doping. Undoped and Mn-doped ZnS films on glass substrates exhibit preferred cubic (111) orientation at the substrate temperature of 280°C. The Δ2θ value (full width at half-maximum) of the 111 reflection of the ZnS:Mn film in an X-ray diffraction pattern was found to be improved with an increase in Mn concentration and film thickness. By means of secondary ion mass spectroscopy (SIMS) and electron spin resonance (ESR) analyses, it was confirmed that this doping technique was very efficient in obtaining a homogeneous Mn2+ distribution which yielded highly luminescent EL devices. For an optimum device with double insulators of stacked Ta2O5/Al2O3 layers, the maximum luminance and efficiency at 1 kHz sinusoidal excitation were 3200 cd/m2 and 1.3 lm/W, respectively.

Preparation of CdSSe-ZnS superlattice, SrS, and CdSSe-SrS superlattice by hot-wall epitaxy, and applications to electroluminescent devices

CdSSe (manganese-doped, Eg = 1.9–2.5 eV, lattice constant a = 6.05–5.8A)-ZnS (Eg = 3.56 eV, a = 5.41A) superlattices, SrS (cerium-doped, E = 4.4 eV, a = 6.02A) layers, and CdSSe-SrS (cerium-doped) superlattice layers nave been prepared by hot-wall epitaxy, and the properties and the electroluminescent device characteristics of the active layers are reported. For the superlattices with ZnS, the maximum luminance was 800 cd/m2 at an applied sinusoidal voltage (Vo-p = 200 V) with frequency 1kHz, and the wavelength of the spectral peak was 610 nm due to the large strain caused by the lattice mismatch (8–15%) between the CdSSe and ZnS layers. The maximum luminance and Comisson Internationale de Enluminure (CIE) chromaticity of CdS(Mn)-ZnS superlattices and CdSe(Mn)-ZnS superlattice devices were 557cd/m2 and (x,y) = (0.58,0.41) and 982 cd/m2 and (0.61, 0.38), respectively. For superlattices with SrS, the maximum luminance of the device with the SrS (cerium-doped) active layer was nearly 700 cd/m2 at a voltage of 340V. Blue electroluminescent emission was observed in the photon wavelength region less than 450 nm, due to carriers dropping into the quantum wells of the device with the CdSSe-SrS superlattice active layer.