Carole Balloud

Investigation of 2 inch SiC layers grown in a resistively-heated LP-CVD reactor with horizontal “hot-walls”

With respect to more standard and more widely used inductive-heating, the resistivelyheated reactors offer the strong advantage of low cost, easy installation and low running constraints. Combined with an easy adaptation to the increasing size of wafers, this results in very strong advantages. This simple technique was mainly restricted to the growth of small size samples for academic purpose [1]. In this work we report an investigation of 2 inch SiC layers deposited in a new, horizontal and resistively-heated, “hot-wall” LP-CVD reactor specially designed for large flexibility. Introduction Due to superior physical properties, SiC appears as a most promising material for high power, high frequency and high temperature electronic devices or sensors. In this case, whatever is the targeted application, one needs to deposit a low doped, electronic grade material, on a large diameter single crystalline wafer. A promising technique is the use of hot-wall CVD, coupled with resistive heaters. With respect to the more standard and widely used inductive-heating technology, a resistivelyheated reactor offers advantages in terms of (low) cost, (easy) installation and (low) running constraints. Combined with a very easy adaptation to the increasing size of wafers, this seems very appealing. Unfortunately, up to now, this simple technique has been restricted to the growth of small size samples for academic purpose [1] and was not seriously considered for industrial applications. In order to establish more the potentiality of this system, we report a detailed investigation of the thickness uniformity, surface morphology and low temperature photoluminescence properties of a series of epitaxial layers deposited on silicon. We show that, on 2” Si substrates, state of the art material can be easily obtained. Experimental The reactor has been specially designed to allow, both, usual SiC growth by LP-CVD [1] as well as full Si-wafer conversion by LPE [2]. In this case, the control of the vertical temperature gradient is essential to insure an optimised liquid phase diffusion. This technical constraint dictated the choice of two independent heaters (upper and lower resistive) forming then a standard“hot-walls” configuration. The walls are thin (0.5mm) and made of high purity graphite sheets. The thermal inertia is then very low, which allows RTP (Rapid Thermal Processing) to be done up to 1800°C. In order to Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 273-276 doi:10.4028/www.scientific.net/MSF.457-460.273

Hexamethyldisilane/propane versus silane/propane precursors: application to the growth of high-quality 3C–SiC on Si

From a comparative evaluation of hexamethyldisilane (HMDS) and silane–propane (SP) precursor systems, it is shown that HMDS needs a small addition of propane to deposit heteroepitaxial layers of 3C–SiC on Si with superior crystalline properties. In this case, propane compensates for the secondary reactions induced by hydrogen reacting with carbon. Using atmospheric pressure chemical vapour deposition conditions, the new system (HMDS–propane) demonstrates several advantages. It is safer to handle than SP and allows a higher growth rate (up to 7 µm h−1 at 1350 °C) without any degradation of the layer morphology. However, when lowering the deposition temperature, HMDS is revealed to be more stable than silane. This is in contrast to most standard beliefs but explains why a high temperature (~1350 °C) is always necessary to grow high-quality material using HMDS.

Characterization of Bulk 3C-SiC Single Crystals Grown on 4H-SiC by the CF-PVT Method

Because of the formation of DPB (Double Positioning Boundary) when starting from a hexagonal <0001> seed, DPB-free 3C-SiC single crystals have never been reported up to now. In a recent work we showed that, using adapted nucleation conditions, one could grow thick 3C-SiC single crystal almost free of DPB [1]. In this work we present the results of a multi-scale investigation of such crystals. Using birefringence microscopy, EBSD and HR-TEM, we find evidence of a continuous improvement of the crystal quality with increasing thickness in the most defected area, at the sample periphery. On the contrary, in the large DPB-free area, the SF density remains rather constant from the interface to the surface. The LTPL spectra collected at 5K on the upper part of samples present a nice resolution of multiple bound exciton features (up to m=5) which clearly shows the high (electronic) quality of our 3C-SiC material.

Comparative Studies of 4H-SiC Layers Grown with Either Silane or HexaMethylDisilane / Propane Precursor Systems

We compare two series of 4H-SiC layers grown with either silane/propane or hexamethyldisilane/propane precursor systems. In both cases, the growth rate increases with precursor flow. However it saturates and, then, tend to decrease at high temperature. The range of growth conditions (C/Si ratio, growth rate, growth temperature) which give good surface morphology has been studied. The operating windows are identical for the two systems .In both cases, micro-Raman and LTPL spectroscopy confirm the formation of high quality 4H-SiC polytype.

Dilute Aluminium Concentration in 4H-SiC: from SIMS to LTPL Measurements

From a systematic comparison of SIMS measurements with low temperature photoluminescence data, we show that one can directly perform a non destructive (optical) investigation of the residual Al content in 4H-SiC samples. This technique offers the non negligible advantage to be easily extended to Al concentrations below the SIMS detection limit.

Growth of Cubic Silicon Carbide Crystals from Solution

Cubic-silicon carbide crystals have been grown from solution by using the traveling-zone method. In this technique a molten silicon zone heated by induction coils is held between two rods of polycrystalline silicon carbide. Due to the growth set-up and boundary conditions, different mass transfer mechanisms are operative : diffusion, buoyancy, Marangoni convection and forced convection. The growth experiments have been performed on various seed crystals. Cubic SiC crystals were grown with a [111] habit on the [0001] silicon faces of 4H SiC seeds. The polytype 3C-SiC was identified by Transmission Electron Microscopy. Micro Raman spectroscopy and photoluminescence analyses showed good crystalline quality with few 6H inclusions.

Investigation of Thick 3C-SiC Films Re-Grown on Thin 35 nm "Flash Lamp Annealed" 3C-SiC Layers

We report on 3 μm thick 3C-SiC films re-grown on 35 nm thin 3C-SiC layers on Si annealed by a flash lamp process (FLP). Firstly a 35 nm thick 3C-SiC film is deposited on a <100>Si substrate which is followed, secondly, by FLP. This leads to a fast melting (within 20 msec) of the Si-SiC interface region followed by epitaxial solidification. Then, in a third step, this film is used as a seed for the deposition of a second 3 μm thick 3C-SiC layer. This newly developed process including FLP is called FLASiC (Flash Lamp Annealed SIlicon Carbide). Compared with standard 3 μm thick layers directly grown on silicon, both transmission electron microscopy and low temperature photoluminescence evidence improvement of the re-grown material.

Optical Investigation of Stacking Faults and Micro-Crystalline Inclusions In-Low-Doped 4H-SiC Layers

We report a LTPL (Low Temperature PhotoLuminescence) investigation of nominally undoped 4H-SiC epitaxial layers grown on (0001) 4H-SiC substrates. From room temperature Raman spectra, we find evidence of 3C micro-crystalline inclusions (MCIs). From LTPL spectra collected at various distance from the MCIs we find a coexistence of various defects, which behave like quantum wells (QWs) with various effective thickness. Using a 2-dimensional QW approximation we deduce the extent of the perturbation associated with the different defects.

Comparative Evaluation of Free-Standing 3C-SiC Crystals

The evolution of defects versus thickness has been investigated in three different freestanding 3C-SiC samples, using TEM (Transmission Electron Microscopy) and LTPL (Low Temperature Photo-Luminescence) spectroscopy. In all samples, the stacking fault density reduces rapidly within the first 20 µm of the growth. Then it remains constant, at about 5x103 cm-1 up to the end. This behavior is attributed to the easy generation of stacking faults, even under a very low thermal stress, as in-situ experiments reveal. On the opposite the elimination of inversion domains, by bending boundaries during the growth, is found to be sample dependant. This is in good agreement with LTPL results.

Growth and Characterisation of Heavily Al-Doped 4H-SiC Layers Grown by VLS in an Al-Si Melt

We report on a new approach in which the homoepitaxial growth of 4H-SiC layers can be done at low temperature (1100°C). The process involves a VLS (Vapor-Liquid-Solid) mechanism in which propane feeds an Al-Si droplet. Compared to conventional LPE (Liquid Phase Epitaxy) this approach has several advantages. No thermal gradient (vertical or radial) has to be controlled and the liquid can be easily removed before cooling by sucking up the melt. In this way, Al concentrations in the range ~ 1-3.10 20 at.cm -3 have been reached and, both, Raman and TEM characterizations show that no foreign polytype or other material inclusion is formed. The main drawback is a large step bunching, typical of LPE samples.