T. Tuomi

Dislocation evolution in 4H-SiC epitaxial layers

4H-SiC commercial wafers and sublimation grown epitaxial layers with a thickness of 100 μm have been studied concerning crystalline structure. The substrates and the epitaxial layers have been separately investigated by high-resolution x-ray diffraction and synchrotron white beam x-ray topography. The results show that the structural quality was improved in the epitaxial layers in the [1120] and [1100] directions, concerning domain distribution, lattice plane misorientation, mosaicity, and strain, compared with the substrates. Misoriented domains have merged together to form larger domains while the tilt between the domains was reduced, which resulted in nonsplitting in diffraction curves. If the misorientation in the substrate is large, we can only see a slight decrease in the misorientation in the epilayer. At some positions on the substrates block structures (mosaicity) were observed. ω-rocking curves showed smaller full width at half maximum values and more uniform and narrow peaks, while the curvat...

Properties and origins of different stacking faults that cause degradation in SiC PiN diodes

The electrical degradation of 4H–SiC PiN diodes has recently attracted much interest and is a critical material problem for high power applications. The degradation is caused by stacking faults observed as an increased forward voltage drop after forward injection operation. In this article we have combined electrical, optical, and structural techniques to study the formation and growth of the stacking faults causing degradation. We will show three different sources causing two different types of stacking fault properties.

Structural defects in electrically degraded 4H-SiC p+/n−/n+ diodes

Triangular structural defects are occasionally generated during the long-term operation of 4H-SiC pin diodes and degrade the forward characteristics of the diode. We have used synchrotron white beam x-ray topography, scanning electron microscopy, in situ cathodo luminescence, and transmission electron microscopy to characterize the structure and formation of these defects. It is shown that the defects are stacking faults on the (0001) basal planes, bound by partial dislocations with Burgers vectors 1/3〈1010〉 and 1/3〈0110〉. These partials are suggested to form by the dissociation of existing dislocations.

Crystalline imperfections in 4H SiC grown with a seeded Lely method

Abstract Commercially available 4H SiC wafers have been studied concerning their crystal quality. A variety of structure sensitive techniques has been utilized to reveal specific macro-defects in the material. Microscopy examination combined with preferential chemical etching have imaged dislocation networks, micropipes, basal plane defects and cracks. Synchrotron X-ray topographs have shown defect-associated strain and lattice misorientation arising in the vicinity of some micropipes. High resolution X-ray diffractometry and Bragg angle topography were used to provide evidence of existing domains and their misorientation. The results obtained are discussed in the context of defect origin and formation mechanisms. A comparison with 6H SiC is made to derive possible similarities of defect appearance in both polytypes.

Self-organized InAs islands on (100) InP by metalorganic vapor-phase epitaxy

Abstract The effects of growth temperature, InAs deposition thickness and deposition rate on the areal density, size, uniformity and spatial distribution of self-organized InAs nanoscale islands grown on exact and vicinal (100) InP substrates by metalorganic vapor-phase epitaxy are investigated in detail by AFM. At 500°C, the island density is found to increase as the InAs deposition thickness is increased, while the average island size decreases slightly. At growth temperatures above 500°C, larger inhomogenous islands also appear. Decreasing the deposition rate increases the island density and substrate coverage. The unintentional As/P exchange is found to have a significant influence on island formation by producing excess material for the islands. Low-temperature photoluminescence from the recombination of carriers in the buried InAs islands is observed in the 1.4 – 1.8 μm spectral region.

Photoluminescence of Cd1—xZnxTe Crystals Grown by High‐Pressure Bridgman Technique

Photoluminescence (PL) spectra of detector-grade Cd 1-x Zn x Te crystals with x = 0.07 to 0.14 were measured as a function of the excitation laser power and over the temperature range from 12 to 295 K. Two bound exciton transitions at the energies of 13 and 24 meV below the band-gap E g and a number of phonon replicas dominate the low-temperature PL spectra. The transition at the energy of E g - 13 meV is attributed to a neutral donor-bound exciton D 0 X. The excitation power and temperature dependence of the peak at the energy of E g - 24 meV show that it is of excitonic origin with binding energy of 13 meV. The intensity of this A 0 X peak is found to depend on z, which suggests that it is related to structural defects. In some samples a donor-acceptor pair (DAP) transition at the energy of E g - 60 meV is observed. The PL spectra also exhibit a weak, defect-related broad peak at the energy of about E g - 200 meV. The PL of a large number of samples from different boules and locations in the boules is compared. The DAP-peak intensity was observed to depend strongly on the boule.

Effect of growth temperature on the electronic energy band and crystal structure of ZnS thin films grown using atomic layer epitaxy

Electroreflectance (ER) technique has been used to study the effect of growth temperature and source materials on the electronic energy band and crystal structure of zinc sulfide thin films grown using atomic layer epitaxy (ALE). The samples studied were grown by ALE at 300–500 °C from zinc acetate or zinc chloride and hydrogen sulfide. The ER spectra were measured at room temperature and at 77 K. The crystal structure of polycrystalline ZnS thin films grown from zinc acetate at 300–375 °C is mainly cubic, whereas the ZnS layers grown from zinc chloride at 425–500 °C are predominantly hexagonal. The ER spectra of the latter samples show peaks belonging to the cubic structure, too. The crystal structure of zinc sulfide thin films is found to depend both on growth temperature and on source materials.

X-ray diffraction study of microstructure in ZnS thin films grown from zinc acetate by atomic layer epitaxy☆

A structure analysis of ZnS thin films is carried out. The films were grown from anhydrous zinc acetate and hydrogen sulphide by atomic layer epitaxy (ALE) at 290–330°C. The thicknesses of the films range from 110 to 1350 nm. The microstructural parameters are determined by Fourier line profile analysis. The average crystallite sizes are 40–80 nm, depending on the film thickness. The values are more than one-half of those observed earlier for ZnS thin films grown by ALE from zinc chloride and hydrogen sulphide and two to three times those found in the films grown by electron beam evaporation (EBE). The relative microstrain in the films is found to be (1−2) × 10−3, which is greater than that observed for films grown by ALE from zinc chloride, but less than that measured for the EBE films.

Doping-induced strain in N-doped 4H–SiC crystals

Stress in epitaxial layers due to crystal lattice mismatch directly influences the growth, structure, and basic electrophysical parameters of epitaxial films and also to a large extent the degradation processes in semiconductor devices. In this letter, we present a theoretical model for calculating the induced lattice compression due to N doping and the critical thickness concerning formation of misfit dislocations in homoepitaxial 4H–SiC layers with different N-doping levels. For example: The model predicts that substrates with a N concentration of 3×1019 cm−3 induce misfit dislocations when the epilayer thickness reaches ∼10 μm. Also, the N-doping concentration in the 1×1018–1×1019 cm−3 range yields a strain that not will cause misfit dislocactions at the substrate and epilayer interface until an epilayer thickness of 200–300 μm is reached. Supporting evidence of the induced lattice compression due to N doping have been done by synchrotron white-beam x-ray topography on samples with different N-doping l...

Comparative study of the crystal phase, crystallite size and microstrain in electroluminescent ZnS:Mn films grown by atomic layer epitaxy and electron beam evaporation

Abstract The crystallite size, the microstrain and their dependence on the thickness of the active layer in electroluminescent ZnS:Mn thin films were studied by X-ray diffraction line profile analysis. The average crystallite size in the films grown by atomic layer epitaxy (ALE) is about 100 nm, which is larger than that for films deposited by electron beam evaporation (EBE) by a factor of 5–10. In addition, the relative microstrain in the films prepared by EBE was about six times that in the films grown by ALE. The comparison is made using the average crystallite size values for both types of sample. Also the crystal phase and dislocation density were clearly different in the two types of the thin film. It is suggested that the large crystallite size and therefore the low density of crystallite boundaries are very probably causes of the observed increase in electroluminescent efficiency.