Makoto Inagaki

N–H related defects in GaAsN grown through chemical beam epitaxy

The local vibration modes of N–H related defects in GaAsN are studied using isotopes. When GaAsN is grown through chemical beam epitaxy (CBE) using triethylgallium/tris(dimethylamino)arsenic/monomethylhydrazine gas, there are several local vibration modes (LVMs) in Fourier transform infrared (FTIR) spectra. Signals with stretching mode peaks at 2952, 3098, and 3125 cm−1 are reported, along with new wagging and stretching mode peaks at 960 and 3011 cm−1, which exist only in crystals grown through CBE. When the film is grown using deuterated MMHy as a nitrogen source, new peaks at 2206, 2302, 2318, 2245, and 714 cm−1 appear. This suggests that D related defects are created because of the deuterated MMHy. The ratios of frequencies of these new peaks to those obtained from crystals grown using MMHy are nearly 1.34. This suggests that all defects in GaAsN grown through CBE, which appear as LVMs, are N–H related defects. Especially, those with LVMs at 960 and 3011 cm−1 are new N–H defects only found in GaAsN grown through CBE.

Improvements in Optoelectrical Properties of GaAsN by Controlling Step Density during Chemical Beam Epitaxy Growth

Improvements in optoelectrical properties of GaAsN are demonstrated by chemical beam epitaxy (CBE) growth on high-step-density GaAs substrates. The step density at the growing surface is controlled using 2, 4, and 10° off GaAs(001) wafers as substrates. The number of carrier scattering centers induced by N (SCN) in the grown GaAsN films is quantitatively evaluated from the temperature dependence of hole mobility and used as an indicator of film quality. In previous studies, SCN increased with increasing N composition independently of the growth technique used. By CBE with high-step-density substrates, the reduction in SCN is achieved. This method also improves the emission intensity of cathode luminescence.

Effects of source gas molecules on N–H- and N–D-related defect formations in GaAsN grown by chemical beam epitaxy

The formation mechanism of N–H-related defects in GaAsN grown by chemical beam epitaxy (CBE) is studied on the basis of the isotope effects on the local vibration modes (LVMs) originating from N–H. When deuterated monomethylhydrazine (MMHy) is used as the N source, LVM signals from the nitrogen–deuterium bond (N–D) are obtained. However, there are still N–H peaks in the IR absorption spectra, which have intensities similar to those of N–D peaks. When the film is grown with deuterated triethylgallium (TEGa), there are no N–D peaks. The peak intensity at 2952 cm−1 increases with increasing tris(dimethylamino)arsenic (TDMAAs) flow rate, and that at 3098 cm−1 is almost constant regardless of the flow rate. These results indicate that H atoms in the N–H-related defects originate from H directly bonded to N in MMHy and CH3 in MMHy and/or H in TDMAAs, not from TEGa.

High electron mobility in Ga(In)NAs films grown by molecular beam epitaxy

We report the highest mobility values above 2000 cm2/Vs in Si doped GaNAs film grown by molecular beam epitaxy. To understand the feature of the origin which limits the electron mobility in GaNAs, temperature dependences of mobility were measured for high mobility GaNAs and referential low mobility GaInNAs. Temperature dependent mobility for high mobility GaNAs is similar to the GaAs case, while that for low mobility GaInNAs shows large decrease in lower temperature region. The electron mobility of high quality GaNAs can be explained by intrinsic limiting factor of random alloy scattering and extrinsic factor of ionized impurity scattering.

Improvement of minority-carrier lifetime in GaAsN grown by chemical beam epitaxy

Minority-carrier lifetime (electron) in p-type GaAsN film is improved by chemical beam epitaxy (CBE) and thermal annealing. Growth rate (GR) in CBE is an important factor to improve minority-carrier lifetime in bulk (τ<inf>B</inf>). For as-grown samples, τ<inf>B</inf> is increased from 3.2 × 10² ps (GR = 2 µm/h) to 9.0 × 10² ps (GR = 0.4 ×m/h). The obtained τ<inf>B</inf> is much longer than the minority-carrier lifetime (∼ 10¹ ps) predicted by the reported carrier diffusion length and mobility. This improvement is due to the increase of nonradiative recombination lifetime (τ<inf>NR</inf>) caused by the decrease of nonradiative recombination centers. By thermal annealing, PL lifetime (τ<inf>PL</inf>) is increased. Therefore, CBE and thermal annealing have a high possibility to obtain the minority-carrier lifetime required to solar cells (> 1 ns).

Shallow Carrier Trap Levels in GaAsN Investigated by Photoluminescence

Shallow carrier trap levels in GaAs1-xNx (0.0010≤x≤0.0038) were investigated by photoluminescence (PL) and photoreflectance (PR) ranging from 4.2 to 300 K. The band gap energies of the GaAsN were clearly determined in the whole temperature range by the PR fitting analysis. It is clarified by peak decomposing that there were three emission peaks in the near-band-edge PL spectra of GaAsN. One of them was originated from band-to-band transition. The energies of two emission peaks were located at approximately 6 and 17 meV below the band edge. The existence of these peaks is evidence of carrier localization at the near-band-edge. The intensity ratio of the peak at the low energy side to other peaks increases with increasing N composition. This behavior is similar to the degradation of electrical properties.

Fabrication of GaAsN solar cell by chemical beam epitaxy with improved minority-carrier lifetime

A GaAsN solar cell is fabricated by using the chemical beam epitaxy. The properties of the external quantum efficiency and short circuit current density are studied by comparing with the calculated values. The external quantum efficiency of the fabricated GaAsN solar cell was lower in the low wavelength region than the calculated value which does not consider the recombination at the interfaces. That is considered to be the one of the reasons for the low short circuit current density by 15 to 20 percent compared to the calculated value.

Low temperature growth GaAs on GE by chemical beam epitaxy

Hetero-epitaxy of GaAs films on Ge were grown by the chemical beam epitaxy (CBE) technique for super-high-efficiency multi-junction solar cells. However, the growth of III-V on Ge remains a key challenge due to anti-phase domains (APDs) and the inter-diffusion at the interface, resulting in the deterioration of the solar cell efficiency. In order to overcome these problems, we introduced the CBE technique, which was carried out under a higher vacuum (∼10−2 Pa) and at relatively lower temperature (360∼460 ¼) as compared with the growth of GaAs on Ge by the typical metalorganic vapor phase epitaxy (MOVPE). The effects of the low growth temperature, the source flow ratio of [V/III] and pre-annealing temperature on crystal qualities and optical properties of GaAs films on Ge were investigated. The results showed that GaAs films on Ge had a good structural quality at growth temperature from 400 to 460 °C with the [V/III] of flow ratio from 50 to 70 by a high resolution x-ray diffraction. The pre-annealing temperature and time were confirmed to play an important role in the modification of Ge substrate surface. The low-temperature (4.5 K) photoluminescence spectra showed the emission peak around 1.51 eV, corresponding to an exciton bound-to-acceptor, in sample grown at 400 °C and the full width at half maximum of this peak was evaluated 10 meV by Gaussian fitting. An emission peak of 1.27 eV caused by Ge diffusion into GaAs film was observed for the sample grown at 460 °C. Such a critical temperature that activates the Ge diffusion seems relatively low in CBE system as compared with that reported in MOVPE system.

Optical DLTS for the study of recombination centers in GaAsN grown by chemical beam epitaxy

New broad DLTS peak signals in GaAsN solar cell, grown by chemical beam epitaxy, were obtained using the combination of optical-irradiation and conventional Deep Level Transient Spectroscopy (DLTS). Those broad peak signals cannot be detected by conventional DLTS method in the dark. The broad peak signals were overlapped with three deep level states at least and showed the increase of DLTS peak intensity. However, the other deep level state (EV+0.60eV) showed no significant change of DLTS peak signals in the dark and optical excitation. The condition of minority carrier injection by optical irradiation indicated that the mechanism of carrier capture and emission at some deep centers had been changed.

Nitrogen Related Deep Levels in GaAsN Films Investigated by a Temperature Dependence of Piezoelectric Photothermal Signal

The temperature dependences of the piezoelectric photo-thermal (PPT) signals from unintentionally doped p-type GaAsN films grown on semi-insulating GaAs substrate were measured from 80 to 300 K. From the theoretical analysis based on the rate equation for the recombination of photo exited carriers to the localized levels, we identified five majority hole traps, P1-P5 in GaAsN films. Among them, estimated concentrations of the P3 and P5 traps increased with the nitrogen contents. Therefore, we concluded that these two traps were due to nitrogen-related recombination centers in GaAsN.