Enhancement of Material Quality of (Si)GeSn Films Grown by SnCl4 Precursor

Tin-based silicon photonics has been widely explored due to the ability of achieving direct bandgap group IV alloys by incorporating Sn in Ge and SiGe lattices.  Chemical vapor deposition has been one of the major methods for the growth of GeSn ans SiGeSn materials[1-3].  Adoption of a viable Sn precursor has been a challenge for the growth of Sn-based alloys. Introduction of deuterated stannane (SnD₄) has been considered most suitable at the beginning. However, instability and difficulty in handling of the gas promoted the use of SnCl₄ as an alternative precursor[1]. Recent advances in growth of device quality GeSn films using SnCl₄ and GeH₄ has been promising. Nevertheless, a Ge-buffer layer has been required due to the Si-etching characteristic of SnCl₄[2]. Recently, buffer-free growth of GeSn using GeH₄ and SnCl₄has been reported without using a carrier gas[3]. Further enhancement in the material quality is considered essential for device applications.  In this paper we report on the effect of carrier gases to improve film quality in buffer-free (Si)GeSn layers grown directly on Si substrate.

A complete set of experiments has been performed to study the effect of Ar carrier gas at deposition pressures from 0.1 torr to 1 torr and substrate temperature of 300 to 400 ºC. Fig. 1 (a) shows the X-ray diffraction (XRD) pattern from (004) plane of the GeSn and SiGeSn films. After normalizing the Si peaks, the GeSn and SiGeSn peaks were compared. The intensity of the GeSn growth shows an increase by one order of magnitude after decreasing the SnCl₄ flow and adding Ar as the carrier. Reduction in the intensity of SiGeSn film is due to lower mobility of SiH₄on the surface and lower decomposition rate at the growth temperature. Comparison of Fig.1 (a) and (b) shows enhanced crystal quality of the film with Ar.  The asymmetrical reciprocal space map (RSM) XRD scan of the samples from (-2-24) plane show more than 95% strain relaxation . The spread observed in the omega direction is due to the mosaicity of the film as a result of strain relaxation.  The comparison between the RSMs of samples without (d) and with (e) Ar carrier gas confirms higher quality of growth with Ar. Fig.2 shows photoluminescence characterization of the samples. The higher intensity of sample with Ar flow shows higher quality of the samples.

Growth of (Si)GeSn films using SnCl₄ is greatly affected by the role of reaction byproducts during deposition of the films. Etching of the Si substrate with the production of HCl in the GeH₄+SnCl₄ reaction has been the main reason for growth of Ge buffer layer prior to the GeSn growth. In the reported GeSn growth using SnCl₄ on Ge-buffered substrate, different carrier gases, such as, H₂ and N₂ have been used[2]. In order to achieve a direct growth of GeSn films on Si substrates using GeH₄ precursor, SnCl₄has been used without a carrier gas to increase the the activation energy for film etching[3]. Increase in the activation energy of the etching has been at the expense of reduction in the surface mobility of the ad-species on the surface and reduction of film quality. Nonetheless, enhancement of film quality requires introduction of the carrier gas or increase in the growth temperature.

In order to suppression of etching the amount of SnCl₄ flow is decreases while higeher surface mobility of ad-species is promoted by using  Ar as the carrier gas. Usage of H₂ as the carrier gas is avoided as it changes the balance of the GeH₄+SnCl₄ reaction towards production of more HCl which is an etchant of Ge and Si. In addition, N₂has not been as it can reduce the activation energy of growth less than Ar due to the smaller atomic weight. Therefore,  using a larger atom size gas such as Ar is more favorable. Under such conditions, the growth temperature could be increased from 300 to 400C which results in enhanced the optical and material quality of the samples.

In summary, effect of Ar carrier gas in enhancing the quality of (Si)GeSn films grown directly on Si substrate has been investigated.  The results show that using Ar as a carrier gas high quality SiGeSn films with comparable stoichiometry are achieved at lower SnCl₄flow rates and higher (CMOS-compatible) deposition temperatures.

 

References:

[1] Kouvetakis et al. J.Mater. Chem.17, 1649–1655 (2007)

[2] Margetis et al. ECS Transactions, 64, (6) 711-720 (2014).

[3] Mosleh et al. Frontiers in Materials 2, 30 (2015).