Elsevier

Surface and Coatings Technology

Volume 201, Issue 7, 20 December 2006, Pages 4026-4031
Surface and Coatings Technology

Vapor growth of SiC bulk crystals and its challenge of doping

https://doi.org/10.1016/j.surfcoat.2006.08.033Get rights and content

Abstract

The paper reviews the so-called Modified-PVT (M-PVT) technique which combines the state of the art PVT technique for SiC crystal growth with physical and chemical vapor deposition (PVD and CVD) for fine tuning of growth parameters and improved doping. Using this technique, currently the highest aluminum doping levels and lowest resistivity values in p-type bulk SiC were achieved that for the first time meet device fabrication needs. The paper will address fundamentals of the Modified-PVT technique including a comparison of experimental results with numerical modeling of the gas flow. As additional gas feeding helium, helium–aluminum vapor for p-type doping, phosphine for n-type doping and propane for fine tuning of the C/Si gas phase composition will be discussed. So far, the M-PVT concept, i.e. mixture of conventional PVT and fine tuning by PVD/CVD, enables the most flexible doping of SiC single crystals.

Introduction

The wide bandgap semiconductor SiC which may be prepared by physical vapor transport (PVT) growth at elevated temperatures above 2000 °C has gained much interest in recent years for high power, high frequency, high temperature and sensor applications in harsh environments. High and homogeneous doping of the single crystal substrates is a prerequisite to ensure reliable device operation and great device fabrication yield. The paper reviews the so-called Modified-PVT (M-PVT) technique [1], [2] which combines the state of the art PVT technique for SiC crystal growth [3], [4], [5], [6], [7], [8], [9], [10] with physical and chemical vapor deposition (PVD and CVD) for fine tuning of growth parameters and improved doping. Using this technique, currently the highest aluminum doping levels and lowest resistivity values in p-type SiC (ρ = 0.1 Ω cm…0.2 Ω cm) have been achieved that for the first time meet device fabrication needs. In addition, the highest phosphorous concentrations for alternative n-type doping were demonstrated ([P] > 1 · 1018 cm 3). The paper will address three issues: (i) The development of the Modified-PVT technique will be described with emphasis on the benefit of the additional gas pipe that allows the direct control of gas phase composition. Numerical modeling of the impact of the additional gas flow on the conventional PVT sublimation process will be presented. (ii) As additional gas, feeding of (a) inert gas helium, (b) helium–aluminum vapor for p-type doping, (c) phosphine for n-type doping and (d) propane for fine tuning of the gas phase composition, i.e. intention to control the C/Si ratio, will be discussed. (iii) Finally, we will present first investigations using propane for C/Si gas phase composition tuning and discuss non-standard doping of crystals, which reveals a deeper understanding on defect generation during growth.

Section snippets

Modified-PVT technique

As briefly mentioned above, SiC single crystals for commercial applications are fabricated at elevated temperatures above 2000 °C by the so-called physical vapor transport (PVT) method. Growth takes place in a closed graphite crucible. SiC powder which is placed in the hot zone at the bottom of the growth cell (e.g. 2200 °C) sublimes and re-crystallizes in the colder zone (e.g. 2150 °C) in the top area at the seed (Fig. 1, left). Growth control is limited to the setting of (i) growth

Doping of SiC by the M-PVT technique

For n-type doping of SiC by nitrogen the conventional PVT technique is already suitable. In this case nitrogen (N2) gas is used which is inert to graphite and can diffuse through the partially porous crucible wall in the growth chamber (see Fig. 1-left). The situation totally changes for the n-type dopant phosphorus and p-type dopant aluminum where the M-PVT technique is superior due to its direct dopant feeding possibility.

Variation of C/Si ratio by propane feeding

In order to control the C/Si ratio in the gas phase in front of the SiC growth interface we have studied the inlet of silane and propane. In the case of propane the driving force is related to the idea that carbon rich conditions may favor the formation of the 4H–SiC polytype rather than the 6H–SiC counterpart. In first experiments we have feed various amounts of propane gas through the additional gas inlet into the growth cell. Concerning the growing polytype so far no impact of a carbon

Non-standard SiC crystal growth for fundamental defect formation studies

In order to better understand defect formation during SiC crystal growth we have studied the impact of doping on dislocation generation and annihilation. For this reasons we have carried out a special crystal growth experiment where we have switched back and forth between p-type and n-type doping. The experiment makes use of the benefits of the M-PVT setup. We established a continuous aluminum dopant flux for dopant incorporation in the mid-1018 cm 3 range. After one-third of the total process

Summary

The Modified-PVT (M-PVT) technique which combines the state of the art PVT technique for bulk SiC crystal growth and physical/chemical vapor deposition (PVD/CVD) is the currently most feasible method for fine tuning of growth parameters and improved doping. We have shown how numerical modeling may be applied to understand the growth process underlying physical and chemical effects; in particular quantitative agreement between experiment and computer simulation for various gas inlet fluxes has

Acknowledgements

We would like to acknowledge financial support by the Deutsche Forschungsgemeinschaft (contract number WE2107/3) and the Bavarian French University Center (BFHZ). We would like to thank P. Desperrier from the own research team for carrying out the phosphorous doping experiments and Ulrich Starke from the Max Planck Institute in Stuttgart for SIMS measurements on these samples.

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