Koki Saito

Characterization of p‐type GaAs heavily doped with carbon grown by metalorganic molecular‐beam epitaxy

p‐type GaAs with doping levels of up to 5.8×1020 cm−3 has been grown by metalorganic molecular‐beam epitaxy (MOMBE) using carbon (C) as a dopant. The mobility and minority‐carrier diffusion length of the C‐doped MOMBE layers were comparable to those of Be‐doped MBE layers. The diffusion coefficient of C at 900 °C was estimated to be 6×10−15 cm2 /s which is about two orders of magnitude less than that of Be (1×10−12 cm2 /s). In addition, the lattice constant of C‐doped GaAs was found to be 5.6533 A which completely matches that of the substrate, while the lattice constant of Be‐doped GaAs decreases to 5.6467 A at a doping level of 2×1020 cm−3 as reported by Lievin et al. [Inst. Phys. Conf. Ser. No. 79, 595 (1985)].

Metallic p-type GaAs and GaAlAs grown by metalorganic molecular beam epitaxy

Abstract The potential of metalorganic molecular beam epitaxy (MOMBE) for the growth of extremely low resistivity p-tape GaAs and GaAlAs has been investigated. The acceptor is carbon which is incorporated from trimethylgallium. The hole concentration was controlled in the range 10 19 -10 21 cm -3 by changing the growth temperature. The highest hole concentrations of 1.5×10 21 and 2.5×10 21 cm -3 have been obtained for GaAs and GaAlAs with the Al mole fraction of 0.35, respectively, which are higher than any other values reported for p-type epitaxial GaAs and GaAlAs thin films. The growth mechanism and detailed electrical properties have been discussed. It was found that the behavior of carbon as an acceptor is much more suitable than that of beryllium for device applications, which require thin layers with ultra low resistivity.

Heavily carbon doped p-type GaAs and GaAlAs grown by metalorganic molecular beam epitaxy

We have investigated the potential of metalorganic molecular beam epitaxy (MOMBE) for the growth of extremely low resistivity p-type GaAs and GaAlAs. Heavily doped GaAs and GaAlAs epilayers with a carrier concentration of 5 × 1020-1 × 1021cm-3 were successfully grown by MOMBE. The acceptor was carbon which was incorporated from TMG. The thermal diffusion coefficient was much lower than that of Be, and the lattice constant did not vary with increasing carrier concentration. The behavior of carbon as an acceptor is very suitable for device applications, which require thin layers with ultra-low resistivity.

Metallic p-type GaAs and InGaAs grown by MOMBE

Abstract Heavily carbon-doped p-GaAs layers with a hole concentration of 1.5 × 10 21 −3.4 × 10 18 cm -3 were grown by metalorganic molecular beam epitaxy (MOMBE). The carrier concentration agrees well with the carbon concentration measured by SIMS, which suggests 100% of electrical activation of the incorporated carbon as an acceptor. The p-n diodes with carbon-doped p-GaAs show good rectification. The lattice constant of GaAs decreases with increasing carbon concentration. The carbon-doped InGaAs with a hole concentration of 2.6 × 10 20 cm -3 , lattice-matched with a GaAs substrate, was obtained for the first time. The effective bandgap narrowing in heavily doped GaAs was also studied by photoluminescence. The measured bandgap with a hole concentration of 10 20 cm -3 is about 100 meV lower than that of intrinsic GaAs. Finally, the static and high-frequency characteristics of heterojunction bipolar transistors with carbon-doped p-GaAs were calculated.

Characterization of FeSe thin films prepared on GaAs substrate by selenization technique

FeSe thin films were prepared on GaAs(100) substrate by the selenization of Fe films using molecular-beam epitaxy. FeSe compound thin films were obtained at a substrate temperature above 380 °C. From the depth profiles of Fe and Se in the selenized film measured by Auger electron spectroscopy, it was confirmed that an FeSe layer with a constant ratio of Fe/Se was formed. The measured composition ratio of Fe/Se in the film was 1/3. It was different from the composition in Fe3Se4 or Fe7Se8, which is a stable bulk FeSe compound. From the measured M–H curve, it was found that the obtained FeSe film consisted of two phases with different magnetic properties.

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se2 (CIGS) thin-film solar cells using evaporated InxSey and ZnInxSey buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnInxSey and the CdS buffer layers.

Photo atomic layer deposition of transparent conductive ZnO films

Low-resistivity ZnO films were grown by photo atomic layer deposition (photo-ALD) technique using diethylzinc (DEZ) and H2O as reactant gases. Self-limiting growth was achieved for the temperature range from 105°C to 235°C. It was found that UV light irradiation was very effective to increase the electron concentration of the films and the electron concentration of 5 × 1020 cm−3 was achieved even in undoped ZnO. Thus, the resistivity of the films grown with UV irradiation was one order of magnitude less than that grown without UV irradiation. The minimum resistivity of 6.9 × 10−4Ω cm was obtained by photo- ALD method without any intentional doping.

Improvement in Performances of ZnO:B/i-ZnO/Cu(InGa)Se2 Solar Cells by Surface Treatments for Cu(InGa)Se2

Solar cells based on Cu(InGa)Se2 (CIGS) thin films fabricated by selenization/sulfurization were investigated. We have concentrated on studying the heterojunction quality to improve the efficiency and stability of n-ZnO/i-(atomic layer deposition :ALD)ZnO buffer-layer/p-CIGS structure devices. The effect of heat treatment for CIGS absorbers was studied. It is found that the heat-treatment can remove entities, e.g., excess InxSy, from the surface of CIGSS, which causes interdiffusion in the (ALD)ZnO buffer layer and decreases Voc and FF. We achieved 13.9% efficiency (Voc: 510 mV, FF: 0.736, Jsc: 36.9 mA/cm2) without the use of Cd-related material. Reversible light-soaking and current-injection effects were observed clearly in i-(ALD)ZnO/CIGS-based solar cells. These phenomena can be controlled by changing the CIGS surface conditions. Devices treated with NH4OH as well as deionized water in an ultrasonic bath prior to the growth of buffer layers are not sensitive to light illumination or application of bias voltage in the dark. The origin of the light-soaking/current-injection effect is assumed to be at/near the CIGS surface or grain boundaries.

THEORETICAL ANALYSIS OF HEAVY DOPING EFFECTS ON ALGAAS/GAAS HBT'S

The minority electron transport in p-type GaAs with hole concentrations greater than 1020 cm-3 was investigated using the Monte Carlo method. It is found that electron-hole and the plasmon scatterings play a dominant role in heavily doped p-type GaAs. The estimated electron mobility and diffusion length are in good agreement with the experimentally obtained values. Based on the above, the static and high-frequency characteristics of AlGaAs/GaAs HBTs were analyzed. The band gap narrowing and the Auger recombination are found to induce substantial effects on the current gain. The maximum frequency of oscillation of 440 GHz and the propagation delay time of 1.5 ps/gate are predicted for the base doping of 1×1021 cm-3.

STRUCTURAL AND MAGNETIC PROPERTIES OF FEXSEY THIN FILMS DURING THEIR SELENIZATION PROCESS

FexSey films were prepared on GaAs(001) substrates by a selenization of Fe films using molecular beam epitaxy equipment. Structural and magnetic properties of FexSey thin films during their selenization process were studied. The selenized films obtained consisted of polycrystalline grains of 100–700 nm. A magnetic anisotropy of in-plane/perpendicular to the films was weakened by increasing the selenization ratio of the samples, which was interesting in contrast to the fact that the grain size of the films became larger.