Search

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 B ). For as-grown samples, τ B is increased from 3.2 × 102 ps (GR = 2 µm/h) to 9.0 × 102 ps (GR = 0.4 ×m/h). The obtained τ B 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 NR ) caused by the decrease of nonradiative recombination centers. By thermal annealing, PL lifetime (τ PL ) is increased. Therefore, CBE and thermal annealing have a high possibility to obtain the minority-carrier lifetime required to solar cells (> 1 ns).

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.

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.

Decreasing the growth rate in chemical beam epitaxy (CBE) is effective to improve the hole mobility and minority-carrier lifetime in p-GaAsN films. The hole mobility increased from 120 to 150 cm2 V-1 s-1 for the N composition of 0.6%. The minority-carrier lifetime improved from 3.2×10-1 ([N] = 0.6%) to 9.0×10-1 ns ([N] = 0.8%) despite the higher N composition. N-related scattering centers are indicated to be the dominant scattering centers at approximately room temperature. Controlling the growth rate is considered to be effective to reduce the amount of N-related scattering centers and nonradiative recombination centers.

Decreasing the growth rate in chemical beam epitaxy (CBE) is effective to improve the hole mobility and minority-carrier lifetime in p-GaAsN films. The hole mobility increased from 120 to 150 cm2 V-1 s-1 for the N composition of 0.6%. The minority-carrier lifetime improved from 3.2×10-1 ([N] = 0.6%) to 9.0×10-1 ns ([N] = 0.8%) despite the higher N composition. N-related scattering centers are indicated to be the dominant scattering centers at approximately room temperature. Controlling the growth rate is considered to be effective to reduce the amount of N-related scattering centers and nonradiative recombination centers.

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 (SC N) 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, SC N increased with increasing N composition independently of the growth technique used. By CBE with high-step-density substrates, the reduction in SC N is achieved. This method also improves the emission intensity of cathode luminescence.