K. Higashi

Nitrogen Gas Flow Driven Unintentional Incorporation of Al during the Growth of Dilute Nitride Semiconductor by Plasma-Assisted Molecular Beam Epitaxy

We report the unintentional incorporation of Al during the growth of molecular beam epitaxy using RF plasma source, driven by N2 gas flow. The concentrations of N, Al, O, and C within GaNAs/GaAs/AlAs structure are investigated by secondary ion mass spectrometry. In spite of the closed shutter of Al cell, we observe Al incorporation with a concentration up to 1×1018 cm-3 in GaNAs layer and characteristically in the bottom side GaAs. Its concentration is solely dependent on N2 gas flow rate. Remarkably, the operation of the RF plasma has no impact on that. C and O show their concentrations corresponding to the extrinsic Al. The complex interactions between those elements predict a possible origin of material deteriorations and difficulty for the precise doping control.

Direct observation of N-(group V) bonding defects in dilute nitride semiconductors using hard x-ray photoelectron spectroscopy

Using bulk sensitive hard x-ray photoelectron spectroscopy, we directly observe a spectrum related to N–As bonding defects in (Ga,In)(N,As)/Ga(N,As) heterostructure. The defects are most likely attributed to split interstitials. Their concentration is in the order of 1019 cm−3, close to the detection limit of the measurement. Rapid thermal annealing eliminates the defects, leading to those undetectable. Similar phenomenon is observed for N–P bonding defects in In(N,P). The results indicate common features in dilute nitride semiconductor system: existence of N-(group V) bonding defects and their behavior on postgrowth annealing.

Unintentional Source Incorporation in Plasma-Assisted Molecular Beam Epitaxy

Introduction of nitrogen gas induces inefficient Al beam termination when the shutter closes during molecular beam epitaxy. In spite of the closed shutter of the Al cell, the epitaxial layer contains Al at a concentration (nAl) of over 1018 cm-3. This depends on the vapor pressures of N2 (PN2 ) and Al (PAl) as nAl ∝PN2 PAl. An analytical model considering gas phase scattering reproduce the dependences, suggesting that a large amount of N2 gas causes the scattering of residual Al atoms with an occasional collision resulting in the atoms directed toward the substrate. Possible impacts on the epitaxial layer, caused by the Al incorporation and related phenomena, are also discussed. The scattering of Al, and its getter effect on other elements especially O can cause further undesirable incorporation of impurities within the epitaxial layer.

Coherent growth of GaGdN layers with high Gd concentration on GaN(0001)

We report on the coherent growth of GaGdN with high Gd concentration on a GaN template using radio-frequency plasma-assisted molecular beam epitaxy under elevated growth conditions. X-ray diffraction and cross-sectional transmission electron microscopy observations revealed that at a growth temperature of 700 °C or below, GaGdN layers are coherently grown on the GaN templates without segregation of the secondary phases. As the GdN mole fraction x was increased to 0.08, the c-axis lattice parameter in Ga1−xGdxN increased linearly. Increasing the growth temperature to 750 °C causes lattice relaxation in GaGdN. All GaGdN samples exhibited photoluminescence emissions near the band-edge, a blue luminescence band emission, and a green luminescence band emission. The origin of the green luminescence band emission is discussed in relation to the compressive strain existing in the GaGdN layers coherently grown on GaN.

Strong atomic ordering in Gd-doped GaN

Gd-doped GaN (Ga1−xGdxN) thin films were grown on a GaN(001) template by radio frequency plasma-assisted molecular beam epitaxy and characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM). Three samples with a different Gd composition were prepared in this study: x = 0.02, 0.05, and 0.08. XRD and TEM results revealed that the low Gd concentration GaN possesses the wurtzite structure. On the other hand, it was found that an ordered phase with a quadruple-periodicity along the [001] direction in the wurtzite structure is formed throughout the film with x = 0.08. We proposed the atomistic model for the superlattice structure observed here.

Annealing induced atomic rearrangements on (Ga,In) (N,As) probed by hard X-ray photoelectron spectroscopy and X-ray absorption fine structure

We study the effects of annealing on (Ga0.64,In0.36) (N0.045,As0.955) using hard X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements. We observed surface oxidation and termination of the N-As bond defects caused by the annealing process. Specifically, we observed a characteristic chemical shift towards lower binding energies in the photoelectron spectra related to In. This phenomenon appears to be caused by the atomic arrangement, which produces increased In-N bond configurations within the matrix, as indicated by the X-ray absorption fine structure measurements. The reduction in the binding energies of group-III In, which occurs concomitantly with the atomic rearrangements of the matrix, causes the differences in the electronic properties of the system before and after annealing.

Epitaxial lift-off for sample preparation of x-ray absorption fine structure

We propose a simple sample preparation technique of x-ray absorption fine structure (XAFS) for its application to the individual layer of practical compound semiconductor devices. An epitaxial lift-off process enables the investigation of pure uppermost thin epitaxial layer without containing information of the bottom-side layers as well as substrate. The plain procedure offers smooth thin film with desired thickness preserving its crystallographic structure, suitable for the measurement. We carry out XAFS measurements for 2.0 and 0.2 microm thick GaAs epitaxial layer at transmission and fluorescence mode, respectively. Clear extended-XAFS oscillation is obtained, and the radial distribution function of which deduces accurate first nearest-neighbor Ga-As bond length to be 2.46 A for both the samples. That shows the feasibility of the proposed technique for the analysis of the precise atomic configurations of thin film semiconductors.