Takao Sakamoto

Investigations of photo-association mechanism for growth rate enhancement in photo-assisted OMVPE of ZnSe and ZnS

Abstract The mechanism of the growth rate enhancement by xenon lamp irradiation in organometallic vapor-phase epitaxy (OMVPE) of ZnSe and ZnS II–VI semiconductors has been investigated by irradiated wavelength dependence of the growth rate. The longest wavelengths where the irradiation highly increased the growth rate were about 500 and 335–350 nm for ZnSe and ZnS, respectively, independent of the source materials used and the VI/II flow ratio. These threshold photon energies nearly corresponded to the bandgaps of the epilayers at the growth temperature, and thus we considered that carriers generated at the growing surface promoted the surface reaction, resulting in the growth rate enhancement.

Growth rate enhancement by xenon lamp irradiation in organometallic vapor-phase epitaxy of ZnSe

The growth rate of ZnSe in organometallic vapor-phase epitaxy (OMVPE) using dimethylzinc (DMZ) and dimethylselenide (DMSe) as the source materials was substantially increased by irradiation from a xenon lamp. The growth temperature was reduced from 500°C to 300°C by keeping a constant growth rate of 1~2 µm/h. Promotion of surface reaction due to irradiation was considered as one of the factors for the growth rate enhancement. The density of defects which causes deep level emissions was also successfully reduced. We suggest that this technique is very effective to obtain high-quality epilayers.

OMVPE of Zn-based II–IV semiconductors using methylmercaptan as a novel sulfur source

Abstract The use of a novel sulfur source, methylmercaptan, CH 3 SH (MSH), for OMVPE growth of ZnS and Zn(S,Se) has been developed for the first time. Gas-phase prereaction was not observed, and the growth temperature of ZnS was successfully reduced, compared with growth using dialkyl-compounds. Zn(S,Se) alloy epilayers lattice-matched to GaAs substrates had specular surface morphology, excellent crystallographic properties, and showed strong excitonic emissions but weak intensities of deep level emissions.

Use of methylselenol for organometallic vapor-phase epitaxy of ZnSe

Heteroepitaxial films of ZnSe on GaAs substrates have been grown by atmospheric-pressure organometallic vapor-phase epitaxy using a new selenium source, methylselenol, CH3SeH (MSeH). Gas-phase prereaction with a group-II source material was suppressed compared with hydride sources, e.g., H2Se. The practical growth temperature was 300–400°C, which was sufficiently lower than 500°C used in the growth with dialkyl compounds. The epilayers exhibited a fairly smooth surface morphology and good crystallographic properties. Higher quality epilayers can be expected with source repurification or synthesis via purer chemical processes.

Effects of lattice-matching on doping characteristics of ZnSxSe1−x epitaxial layers on GaAs substrates grown by OMVPE

Effects of lattice-matching on doping characteristics of ZnSxSe1−x epilayers on GaAs were investigated. The epilayers were grown at 470°C by organometallic vapor-phase epitaxy (OMVPE) from dimethylzinc (DMZn), methylmercaptan (CH3SH, MSH), and dimethylselenide (DMSe). Gallium and nitrogen were employed as donor and acceptor dopants, respectively. For the sulfur composition x between 6% and 12%, ZnSxSe1−x epilayers with a thickness of about 1 μm grew coherently onto the GaAs substrate. In this composition region, gallium-doped epilayers showed low resistivity, while those of other sulfur compositions showed high resistivity. Nitrogen as also effectively doped in the region of coherent growth. Defects generated during the lattice relaxation degraded seriously the electrical properties. It has been found that the lattice-matching is an important factor to dope impurities effectively and to control conductivities.

A Novel Fabrication Method for Polycrystalline Silicon Thin-Film Transistors with a Self-Aligned Lightly Doped Drain Structure

We have developed a novel and simple fabrication method for poly-Si thin-film transistors (TFTs) with a self-aligned lightly doped drain (LDD) structure. The feature of this method is the use of lateral etching of an n+ Si gate electrode to make an LDD region. The lateral etching is realized by plasma etching with SF6 gas. It is found that the etching length in the lateral direction, which is equal to the LDD length, is proportional to the overetching time. Hence, the LDD length is controlled precisely by the overetching time. By applying the fabrication method to make poly-Si TFTs, we have succeeded in decreasing the OFF current by more than one order of magnitude without decreasing the ON current, compared with a conventional poly-Si TFT. It is concluded that our method has a tolerance sufficient to fabricate the LDD structure with a high ON/OFF current ratio.

ESR and Optical Studies of CuInS2 Single Crystals

Infrared (IR) absorption and ESR spectra of iron and chromium impurities were studied in CuInS2 crystals. Both iron in its divalent and trivalent valence states and chromium in its divalent state were observed. The charged states of these ions showed a drastic change in accordance with the Fermi level movement in the band gap introduced by vacuum- and sulfur vapor-annealing. The g-values of these divalent ions determined from ESR (gl=2.20 in Fe2+ and =1.935 in Cr2+) were in good agreement with those estimated theoretically using approximate values of crystal-field parameters (Dq~350 cm-1 in Fe2+ and Dq~570 cm-1 in Cr2+) deduced from optical absorption spectra.

Interface Stress at OMVPE-Grown ZnSxSe1-x/GaAs:Cr Heterostructure

The interface stress at OMVPE-grown ZnSxSe1-x/GaAs:Cr (x=0.03~0.14) heterostructures has been investigated using the Cr-related photoluminescence line at 0.839 eV from GaAs:Cr substrates. From analysis of the shift of the Cr-related luminescence line, it has been found that the GaAs substrates at the heterointerface suffer compressive stress almost independent of the composition of ZnSxSe1-x epitaxial layers. Such compressive stress at the GaAs substrates can be explained as being due to the difference between the thermal expansion coefficients of ZnSxSe1-x and GaAs; the thermal stress introduced in cooling from the growth temperature (500°C) of ZnSxSe1-x on GaAs.

Strained-layer superlattice buffers for high-quality heteroepilayers of ZnSe on GaAs

Abstract We have investigated the role of [ZnSe-ZnS x Se 1- x ]× n strained-layer superlattice (SLS) buffers for obtaining a high-quality ZnSe epilayer on a GaAs substrate from the dependence of PL properties in the epilayers on SLS buffer structures. The SLS buffers have been demonstrated to suppress the propagation of structural defects such as dislocations from the underlying layer to the upper layer. However, when the strain energy accumulated in the SLSs is too high, the defects are newly generated in the upper layer. It is tentatively suggested that optimized SLS buffers are very useful for realizing higher-quality ZnSe epulayers.