Rocking curve peak shift in thin semiconductor layers

A simple x‐ray diffraction method for determining layer composition and mismatch is by measurement of the separation of peaks in a rocking curve. This method can only be used for layers with a thickness above a certain value. This minimum thickness can be significantly large for layers with a small lattice mismatch as in AlGaAs/GaAs or isoelectronic‐doped III‐V semiconductor layers. We give such an example and show that the interference between the diffraction amplitudes of the thin layer and that of the substrate is responsible for the peak shifting of the layer Bragg peak. When this peak shifting is significant, the kinematical diffraction theory and the peak separation method should not be used for the mismatch measurement, and only the dynamical diffraction theory simulation should be used. We present a criterion on the layer thickness, below which the dynamical theory simulation must be used. This thickness is inversely proportional to the lattice mismatch and does not depend on the diffraction geometry, wavelength, and substrate material

Dynamical x-ray diffraction from nonuniform crystalline films: Application to x-ray rocking curve analysis

A dynamical model for the general case of Bragg x-ray diffraction from arbitrarily thick nonuniform crystalline films is presented. The model incorporates depth-dependent strain and a spherically symmetric Gaussian distribution of randomly displaced atoms and can be applied to the rocking curve analysis of ion-damaged single crystals and strained layer superlattices. The analysis of x-ray rocking curves using this model provides detailed strain and damage depth distributions for ion-implanted or MeV-ion-bombarded crystals and layer thickness, and lattice strain distributions for epitaxial layers and superlattices. The computation time using the dynamical model is comparable to that using a kinematical model. We also present detailed strain and damage depth distributions in MeV-ion-bombarded GaAs(100) crystals. The perpendicular strain at the sample surface, measured as a function of ion-beam dose (D), nuclear stopping power (Sn), and electronic stopping power (Se) is shown to vary according to (1–kSe)DSn and saturate at high doses

Strain/Damage in Crystalline Materials Bombarded by MeV Ions: Recrystallization of GaAs by High-Dose Irradiation

MeV ion irradiation effects on semiconductor crystals, GaAs(100) and Si (111) and on an insulating crystal CaF_2(111) have been studied by the x-ray rocking curve technique using a double crystal x-ray diffractometer. The results on GaAs are particularly interesting. The strain developed by ion irradiation in the surface layers of GaAs (100) saturates to a certain level after a high dose irradiation (typically 10^(15)/cm^2), resulting in a uniform lattice spacing about 0.4% larger than the original spacing of the lattice planes parallel to the surface. The layer of uniform strain corresponds in depth to the region where electronic energy loss is dominant over nuclear collision energy loss. The saturated strain level is the same for both p-type and n-type GaAs. In the early stages of irradiation, the strain induced in the surface is shown to be proportional to the nuclear stopping power at the surface and is independent of electronic stopping power. The strain saturation phenomenon in GaAs is discussed in terms of point defect saturation in the surface layer. An isochronal (15 min.) annealing was done on the Cr-doped GaAs at temperatures between 200° C and 700° C. The intensity in the diffraction peak from the surface strained layer jumps at 200° C < T ≤ 300° C. The strain decreases gradually with temperature, approaching zero at T ≤ 500° C. The strain saturation phenomenon does not occur in the irradiated Si. The strain induced in Si is generally very low (less than 0.06%) and is interpreted to be mostly in the layers adjacent to the maximum nuclear stopping region, with zero strain in the surface layer. The data on CaF_2 have been analysed with a kinematical x-ray diffraction theory to get quantitative strain and damage depth profiles for several different doses

Ion Beam Damage in CaF_2

The change in lattice parameter and the induced damage are studied in single crystal CaF_2 bombarded by a 15 MeV Cl ion beam. The lattice parameter change (strain) and the damage for increasing ion beam dose (5 x 10^(12) /cm^2 to 7 x 10^(15)/cm^2) is observed via x-ray rocking curve analysis using a double-crystal diffractometer and x-ray reflection topography. The ion beam energy (range = ~ 4.5 µm in CaF_2) is such that both the electronic region and the nuclear cascade region of energy loss show up in the diffraction signal. By kinematical x-ray diffraction theory analysis, the progress of strain/damage depth profile with increasing beam dose is shown explicitly. The increase in strain is nonlinear with beam dose for the dose range studied. For increasing beam dose, the strain level in the electronic energy loss region is fixed, while that in the nuclear collision loss region increases effectively until that region becomes completely amorphous

MeV Ion Damage in III-V Semiconductors: Saturation and Thermal Annealing of Strain in GaAs and GaP Crystals

MeV ion irradiation of GaAs crystals at room temperature has shown that the lattice strain perpendicular to the sample surface saturates to ~0.47% for cut and ~0.3% for and cut crystals with zero parallel strain in all cases. In this paper, the thermal recovery behavior of the saturated strain in GaAs (100) is presented for a 15 min isochronal annealing. The recovery of strain depth profile is shown explicitly by a dynamical theory analysis of the x-ray rocking curves taken after each annealing step. The isochronal recovery behavior of strain suggests that a spectrum of activation energies is involved in the thermal migration of defects in the saturated surface layer. This also suggests that many kinds of antisite defect complexes exist in the surface layer. The strain and related defects are also shown to saturate in MeV ion bombarded GaP (100) crystals. This may indicate that all the primary defects (interstitials, vacancies, and antisite defects) saturate under MeV ion irradiation of III-V compounds, and support the proposed ion-lattice single collision model of defect production and saturation under MeV ion irradiation. The linewidths of x-ray rocking curves obtained from GaP crystals bombarded at room temperature and at ~490 K indicate that low- temperature recovery stage defects cause major crystal distortion in III-V compounds. Also presented are the isochronal annealing behaviors of lattice strain, x-ray broadening, and peak reflecting power of room temperature irradiated GaP (100) crystals

Ionizing beam-induced adhesion enhancement and interface chemistry for Au-GaAs

MeV ion beam-induced adhesion enhancement of Au-films (∼500 Å thick) on p-type and n-type GaAs substrates has been studied by the scratch test, ESCA, and nuclear reaction hydrogen profiling. For films resistively deposited in a diffusion pumped chamber at 2−5×10^(−6)torr, the data indicate that the adhesion enhancement is associated with oxide layers on the substrate surface adsorbed before the film deposition. The ESCA data suggest that water vapour dissociates and forms Ga(OH)_3 at the interface layers under ionizing radiation. The oxide concentration at the interface varies with substrate electronic properties and gives a large difference in the adhesion enhancement. However, the data obtained so far on the hydrogen concentration at the interface indicate that within our range of sensitivity it is about the same for substrates with different electronic properties. Our data demonstrate the importance of a thin absorbed (impurity) layer for the interface chemistry and adhesion enhancement by ionizing radiation

Two types of MeV ion beam enhanced adhesion for Au films on SiO_2

The ion beam-enhanced adhesion of thin Au films on vitreous silica substrates was studied for a wide range of Cl ion beam doses for beam energies between 6.5 MeV and 21.0 MeV. Since the residual adhesion of Au on SiO_2 is low, the improved adhesion can be easily seen using the Scotch Tape Test. The threshold in the enhanced adhesion corresponding to passing the tape test occurs at two different dose ranges for a given energy; one at very low dose centered around 1 × 10^(13) /cm^2, the other at higher doses with a threshold of around 1.5 × 10^(14) /cm^2 (depending upon the beam energy). At low doses (2 × 10^(12) to 5 × 10^(13) /cm^2) surface cracks occur on the SiO_2 substrates, these cracks close up at doses higher than 5 × 10^(13) /cm^2. A possible explanation of enhanced adhesion in the low dose range is associated with the surface crazing of the SiO_2 substrate. To make the adhesion test more quantitative, a scratch test was also used on the samples