Basic electronic and optoelectronic properties of threading dislocations in heteroepitaxial diamond
Final Report Abstract
For the rapidly increasing activities to develop competitive electronic devices based on diamond, the minimal knowledge on the properties of dislocations is a crucial obstacle. Therefore, we explored in this project the electronic and optoelectronic properties of threading dislocations in diamond using our concept of heteroepitaxial diamond growth on Ir/YSZ/Si(001). We grew high purity single diamond wafers with a thickness 2.2 and 4.4 mm. In crystals cut from the wafers, the dislocation density ndis continuously decreased from initially > 10^10 cm-2 after nucleation to < 10^7 cm-2 at the growth surface. By horizontal slicing, 4 or 8 diamond plates with a thickness of 300 µm were prepared. Each slice had a different ndis. By vertical slicing, cross section probes were manufactured with ndis varying systematically along one edge over the whole range. Pump-probe experiments with the light induced transient free carrier grating (LITG) technique performed on cross section samples revealed a critical value ndis ≈ 10^7cm-2 at which dislocations had a negligible influence on the ambipolar carrier diffusion D. In crystal regions of high ndis (10^9 cm-2), D is reduced by a factor of ≈ 2.5 at low excitation density (<10^15 cm-3), while the opposite behavior, i.e. an increase, is observed at high excitation density (2×10^17 cm-3). The latter is conclusively explained by the splitting of excitons into individual carriers (which have a higher diffusivity) in the inhomogeneous stress field around each dislocation. In corresponding induced absorption (IA) measurements, carrier lifetime ζ scales ∝ 1/ndis over 2 orders of magnitude (10 ns – 0.1 ns) revealing dislocations as dominating recombination centers. Detector crystals were manufactured from all cross section samples so that the transient current technique (TCT) could be used to derive charge collection efficiencies (CCE), separately for electrons and holes. Electrons showed a strong trapping systematically increasing with ndis which facilitated the derivation of absolute capture cross sections. In contrast, trapping for holes was weaker by roughly an order of magnitude scaling less systematically with ndis. In x-ray induced photocurrent (PC) measurements, the samples behaved completely different. The two sets of samples, differed drastically in photoconductive gain G (5-6 vs. 300 at U = ±300 V) while variations were low within each set. Thus, ndis apparently played a minor role. For a further crystal with increased dark conductivity, a G value of G ≈ 7.5×103 at U = ±50 V was obtained. The huge systematic differences for crystals which were all grown nominally identical in high-purity methane/hydrogen gas compositions, suggested that traces of chemical impurities on levels < 1 ppb could now dominate the electronic behavior. As a consequence, we developed a numerical model based on defect free diamond crystals with nitrogen and boron (in concentrations NN and NB) as the only impurities. This model facilitated the calculation of G. The simulations revealed G varying over 6 orders of magnitude thus covering the full range of values formerly reported in numerous publications. Furthermore, the compensation ratio R = (NN-NB)/NB was identified as the most crucial parameter. For a comparison with experiments, NB was determined to 0.3 – 1 ppb by cathodoluminescence (CL). High sensitivity electron paramagnetic resonance (EPR) measurements indicate a similar range for the nitrogen. The highest gain value was interpreted in terms of a nitrogen concentration being too low for a full compensation of boron. Under these conditions our model predictions agree quite well with the experiments. In contrast, charge transport and G values in low gain samples are dominated by structural defects which are not yet included in our model. Thermally stimulated current (TSC) data reveal differences in the traps for different samples. Finally, we could show that a sandwich consisting of a surface acoustic wave (SAW) structure and a high gain diamond crystal can be used as a detector for ionizing radiation (x-, γ−rays).
Publications
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Charge carrier trapping by dislocations in single crystal diamond. Journal of Applied Physics, 127(12).
Schreck, M.; Ščajev, P.; Träger, M.; Mayr, M.; Grünwald, T.; Fischer, M. & Gsell, S.
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Detection of x rays by a surface acoustic delay line in contact with a diamond crystal. Applied Physics Letters, 118(13).
Topaltzikis, Dimitrios; Wielunski, Marek; Hörner, Andreas L.; Küß, Matthias; Reiner, Alexander; Grünwald, Theodor; Schreck, Matthias; Wixforth, Achim & Rühm, Werner
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Growth of Single Crystal Diamond Wafers for Future Device Applications. Wide Bandgap Semiconductors for Power Electronics, 583-631. Wiley.
Schreck, Matthias
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Photoconductive gain in single crystal diamond detectors. Journal of Applied Physics, 129(12).
Grünwald, Theodor & Schreck, Matthias
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DGKK-Fokus: Einkristalliner Diamant – vom exotischen Schmuckstein zum industriell verfügbaren Hochleistungsmaterial, DGKK-Mitteilungsblatt Nr. 113/2022, p. 6 – 8.
Matthias Schreck
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German Patent Application: 10 2022 205 081.9 “Verfahren, Substrat, Vorrichtung und deren Anwendung zur Synthese von ebenen einkristallinen Diamanten“ (Filed 20.05.2022)
Matthias Schreck
