Keigou Maejima

Polarization-induced two-dimensional electron gases in ZnMgO/ZnO heterostructures

Both the formation mechanism and the origin of the two-dimensional electron gas (2DEG) in ZnMgO/ZnO heterostructures have been investigated. The 2DEG in the heterostructures was confirmed to originate from polarization-induced charge and was found to be dominant for transport at low temperatures as well as room temperature (RT) by transport measurements. The origin of 2DEG was concluded to be the surface of the ZnMgO layer based on both capacitance-voltage measurements and the dependence of the carrier concentration on the ZnMgO layer thickness. The largest sheet carrier concentration was 1.1×1013 cm−2 and the highest mobility for the heterostructure was obtained for a Mg composition of 0.61 at RT.

Improvements of ZnO qualities grown by metal-organic vapor phase epitaxy using a molecular beam epitaxy grown ZnO layer as a substrate

Zinc oxide (ZnO) of high quality was homoepitaxially grown by metal-organic vapor phase epitaxy (MOVPE) on molecular beam epitaxy (MBE)-grown ZnO layers after the pretreatment of the underlying MBE-ZnO at 1000°C in N2 which resulted in an atomically flat surface. In photoluminescence at 15 K, the 3 meV line width of the emission from donor-bound-excitons (D0X) and the observation of the fourth phonon replica of the emission from free-excitons (EX) have demonstrated the high potential of MOVPE growth of ZnO toward optical applications.

Strong excitonic transition of Zn1−xMgxO alloy

A strong excitonic optical transition in a Zn1−xMgxO alloy grown by radical source molecular beam epitaxy was observed using both optical reflectivity measurements and photoluminescence (PL) measurements. Clear and strong reflectance peaks at room temperature (RT) were observed from 3.42eV (x=0.05)to4.62eV (x=0.61) from ZnMgO layers at RT. Distinct clear PL spectra at RT were also observed for energies up to 4.06eV (x=0.44). The peak intensity of the reflected signal increased for x values up to x∼0.2 simultaneously with an increase in PL intensity; however, a Stokes shift between the reflectance peak and the PL peak was not observed for x values below 0.2. These facts suggest that the oscillator strength of ZnMgO is enhanced by alloying, and the underlying mechanism is discussed. Furthermore, we demonstrate that the strong reflectance properties even at RT provide an easy method to determine the Mg composition of a thin ZnMgO layer in a ZnMgO∕ZnO heterostructure.

High-efficiency amorphous silicon solar cells: Impact of deposition rate on metastability

Hydrogenated amorphous silicon (a-Si:H) films, used for light absorbers of p-i-n solar cells, were deposited at various deposition rates (Rd) ranging over two orders of magnitude (Rd ∼ 2 × 10−3–3 × 10−1 nm/s) by using diode and triode plasma-enhanced chemical vapor deposition (PECVD). The impact of varying Rd on the light-soaking stability of the solar cells has been investigated. Although a reduction of Rd mitigates the light-induced degradation in the typical range of Rd (>10−1 nm/s), it remains present even in the very low Rd (<10−2 nm/s), indicating that the metastable effect persists in a-Si:H regardless of Rd. The best performing cell, whose a-Si:H absorber is characterized by low amount of metastable defect and high bandgap, can be obtained at Rd of ∼1–3 × 10−2 nm/s by triode PECVD. By applying such a-Si:H in the improved p-i-n devices, we demonstrate two record independently confirmed stabilized efficiencies of 10.22% for single-junction and 12.69% for a-Si:H/hydrogenated microcrystalline silicon ...

Growth of ZnO nanorods on A-plane (1120) sapphire by metal-organic vapor phase epitaxy

The growth mode of ZnO on a-plane (110) sapphire in metal-organic vapor phase epitaxy exhibited stronger tendency toward three-dimensional nucleation at lower temperature (≤700°C) and toward two-dimensional layered growth at higher temperature (900°C), resulting in the formation of rods and films, respectively. ZnO nanorods with diameters smaller than 10 nm were fabricated at the appropriate precursor flow rate and growth temperature. Blue shift of emission from free excitons in a photoluminescence spectrum was observed for the nanorods, particularly for those grown at 400°C. Because the diameters of those nanorods were sufficiently small for quantum confinement, the blue shift was reasonably attributed to quantum size effects.

Band profiles of ZnMgO/ZnO heterostructures confirmed by Kelvin probe force microscopy

The band profiles of ZnMgO/ZnO heterostructures were confirmed through surface potential measurements by Kelvin probe force microscopy. A simple model for the band profile was proposed and the various band parameters were evaluated experimentally and theoretically based on the band model. The band profile was calculated and validated with experimental results using the Schrodinger–Poisson equation. The energy level of the ZnMgO surface donor state, which serves as the source of the two-dimensional electron gas in ZnMgO/ZnO heterostructures, was estimated from the band parameters; nearly identical energy levels around 0.8 eV were obtained for Zn1−xMgxO layers with Mg compositions x ranging from 0.12 to 0.42 and the corresponding charge densities were estimated to be 8×1012 cm−2.

High-efficiency thin-film silicon solar cells realized by integrating stable a-Si:H absorbers into improved device design

We report that thin-film silicon solar cells exhibiting high stabilized efficiencies can be obtained by depositing hydrogenated amorphous silicon (a-Si:H) absorbers using triode-type plasma-enhanced chemical vapor deposition. The improved light-soaking stability and performance of solar cells are also realized by optimizing the device design, such as p and p–i buffer layers. As a result, we attain independently confirmed stabilized efficiencies of 10.1–10.2% for a-Si:H single-junction solar cells (absorber thickness: ti = 220–310 nm) and 12.69% for an a-Si:H (ti = 350 nm)/hydrogenated microcrystalline silicon (µc-Si:H) tandem solar cell fabricated using textured SnO2 and ZnO substrates, respectively. The relative efficiency degradations of these solar cells are ~10 and 3%, respectively, under 1 sun illumination at 50 °C for 1000 h.

Optical dielectric constant inhomogeneity along the growth axis in ZnO-based transparent electrodes deposited on glass substrates

Simple optical measurements using a conventional spectrometer have revealed that in heavily doped ZnO films deposited on glass, the optical dielectric constant is inhomogeneous along the growth axis. Analyses based on Drude’s model have suggested that the origin of this inhomogeneity is the shorter carrier scattering time τc in the portion in contact with the glass substrate, while τc in the major portion of the films has been estimated to be as long as 1×10−14 s at infrared frequencies. This may imply that if better crystallinity is achieved in the initial stage of film deposition, higher conductivity will be attainable without sacrificing the high transparency of the electrode-on-glass system. In addition, the analyses have disclosed the correlation between the high transparency of the major portion of the films and the frequency dependence of τc that is caused by the dopants (Ga or Al) and makes τc even longer at visible-light frequencies.

Quantum chemical study on interactions of diethylzinc with nitrous oxide and water for ZnO growth by metal-organic vapor phase epitaxy

Quantum chemical study on vapor reactions of ZnO growth by metal–organic vapor phase epitaxy (MOVPE) was carried out. Especially, the reaction natures between diethylzinc (DEZn) and different oxygen sources, nitrous oxide (N2O) and water (H2O), have been comparatively discussed, in terms of the adsorption of oxygen sources to DEZn. The adsorption energy and natural population analysis revealed that H2O attacks Zn 4s orbital of DEZn and interaction between H2O and DEZn becomes strong. On the other hand, N2O does not strongly interacts with DEZn. It is confirmed that the reaction of DEZn and H2O is too strong to control by any external energies, while that of DEZn and N2O is well controllable. The result supports the reaction model proposed from the experiments and it is understood that N2O is a desirable source for well-controlled MOVPE.

Density Functional Theory Study on β-Hydride Elimination as Thermal Decomposition Process of Diethylzinc

The β-hydride elimination of diethylzinc (DEZn) is investigated as a decomposition process of DEZn from the viewpoint of quantum chemistry. There are two steps in the β-hydride elimination of DEZn, as we determined from experimental results. Transition states are fully optimized at each β-hydride elimination step, and the energies of these transition states are about 45 kcal/mol higher than their reactants. The products of the two β-hydride elimination steps have higher energies of about 20 kcal/mol than their reactants. These energy profiles agree well with our experimental results indicating that DEZn is decomposed by the two β-hydride elimination steps at 300 and 650 °C and that such decomposition requires thermal energy for the reactions to proceed. On the other hand, about 55 kcal/mol is required to eliminate an ethyl radical from DEZn. Therefore, the main decomposition process of DEZn is β-hydride elimination.