CMOS Compatible Growth of High Quality Ge, SiGe and SiGeSn for Photonic Device Applications

Epitaxial growth of group IV materials on Si has been widely explored for integration of active and passive opto-electronic devices in integrated circuits. In order to meet the limitations imposed by complementary-metal-oxide-semiconductor (CMOS) technology, low temperature growth (below 450 ºC) has been pursued through different methods [1]. Achieving high quality growth at low temperatures is a challenge due to low decomposition rate of SiH₄ and GeH₄ and lower surface mobilty of adatoms. Propagation of threading dislocations and poor crystallinity of the films deteriorates optical efficiency and electronic transport  properties of the film. Reduction of threading dislocation density of the Ge films have been achieved by different methods such as graded SiGe buffer layers, low-temperature:high-temperature (LT:HT) growth of Ge layer and  annealing at elevated temperatures [2-4]. In this paper, we report on the role of different carrier  gases (Ar, N₂ and H₂) in LT:HT growth of Ge layers by chemical vapor deposition (CVD) to attain low threading dislocation density (TDD) films. The optimized growth conditions have been applied for the growth of SiGe and SiGeSn films using SnD₄ as Sn precursor.   

Propagation of defects in Ge rich films are mainly due to the threading of the screw dislocation throughout the film. Growth of a sacrificial layer like SiGe or LT Ge, results in reduction of threading dislocation through graded lattice change or merging of the defects [5].

The transport and reactions of the precursors are highly affected by the carrier gas in CVD processes. The carrier gases play a significant role in transferring the energy to the precursor molecules in the gas phase as well as affecting the deposition on the surface [6]. Larger gas molecules have higher cross-section with the precursors to transfer the energy and break the bonds before reaching the surface. On the other hand, presence of hydrogen in the precursor gases cause, the surface of the sample to be covered with hydrogen after the ad-species are bonded with the surface. 

  Hydrogen as carrier gas increases the partial pressure of hydrogen which lowers the adsorption of ad-species to and desorption of byproducts from the surface. Ar or N₂ gases lower the partial pressure of hydrogen and change the balance in favor of ad-species adsorption and hydrogen desorption during deposition. This results in a lesser competition between ad-species adsorption and hydrogen desorption. 

Fig.1 a shows the X-ray diffraction (XRD) patterns of the Ge films deposited using different carrier gases. The growth was performed using a HT:LT mechanism at 300-550 ºC temperature. The results show that Ge growth in Ar ambient has higher quality in comparison with N₂ and H₂ which is attributed to the larger size of Ar and lower hydrogen partial pressure at the surface. The cross-sectional transmission electron microscopy (TEM) image of growth  with Ar ambient shows TDD as low as 1.3×10⁷ cm^-2 [Fig.1(b)]. Growth in N₂ and  H₂  show poor quality as seen in the XRD patterns [Fig.1(a)].

SiGe and SiGeSn films were grown using the optimized conditions for Ge growth. A complete study of growth from 0.1 torr to 1 torr,  at temperatures from 350-450 ºC for different precursor flow rates is performed. Silicon incorporation in Ge films varied from 1% to 28% for SiGe films [Fig.1(a)]. Tin incorporation of 2% was observed in the films with 13% to 17% Si incorporation [Fig.1(a)]. Investigation of film quality has been done using etch pit measurement with Schimmel solution  for 10 minutes.  The comparison of TDD using TEM (1.3×10⁷ cm^-2) and SEM images (1.7×10⁷ cm^-2) for Ge films confirm the accuracy of the measurements. Figs.1 (c), (d), and (e) show the scanning electron microscopy image (SEM) of etch pit Ge, SiGe and SiGeSn films respectively. The  TDD of SiGe growth is 2.3 ×10⁷ cm^-2_, while the TDD of SiGeSn film is 4.0 ×10⁷    cm^-2. 

Our investigations show that high quality growth of Ge, SiGe and SiGeSn films has been achieved using Ar carrier gas at CMOS compatible temperatures. The material characterization of Ge films show optimized growth condition using Ar carrier gas. Low TDD of SiGe and SiGeSn films show high quality material growth that can be used for optoelectronic device applications.

References:

[1] Mosleh et al. J. Electronic Materials, 43, 4, 938-946,  2014.

[2]  Lieten et al. J. of Crystal Growth, 416, 34–40. 2015.

[3] Sivadasan et al, Eurosoi-Ulis, IEEE conference, 173-176, 2015.

[4] Lee et al, APL Materials, 3, 016102-1 to 016102-7, 2015.

[5] Mosleh et al. ECS Transactions, 64 (6), 895-901, 2014.

[6] Choi et al, Journal of ECS, 148 ,7, C473-C478, 2001.