Sensitivity of Signal-to-Noise Ratio to the Layer Profile and Crystal Quality of SiGe/Si Multilayers

Abstract

This study presents signal-to-noise ratio (SNR) measurements of single crystalline dots or layers of SiGe/Si in multilayer structures in terms of Ge content, interfacial and layer quality. All multilayers were processed in form of mesas and the noise behavior of electrical signal was investigated by comparing the power spectral density curves and K 1/f values. The SiGe/Si multilayer structures were also characterized by the conventional material analysis tools and the results were compared to the noise measurements. The quality of SiGe/Si interface or SiGe layer was monitored by intentional exposure to oxygen in range of 2-1600 nTorr either during or prior to SiGe growth. The results demonstrated that SNR was sensitive to the interfacial and layer quality, and the Ge content in a multilayer structure. SiGe alloys have been widely integrated in large variety of devices such as CMOS for strain engineering, 1-3 bipolar transistors, 4,5 photonic detectors, 6 bolometers, 7-9 quantum cascade lasers 10 and bio-or gas-sensors.11-13 For these applications, SiGe material is grown at low temperatures (350-650• C) where the quality of layers and interfaces plays a critical role in device characteristics. Since high strain amount is often sought then layers with high Ge content have to be grown below the critical thickness in the metastable region. One problem with epi-layers grown at low temperatures is oxygen and moisture contamination which is rooted from epitaxy environment and the purity of the reactant gases. 14,15 An important criterion in all types of devices is the signal-tonoise ratio (SNR) which indicates the quality of the electrical signal. Noise measurement is a sensitive method which can provide accurate information about any minor defects in the crystal.In this article, SiGe material is grown at low temperatures and integrated in a multilayer structure with Si barriers (e.g. SiGe/Si) where the number of periodicity, layer and interface quality were studied by noise measurements. The SiGe material was either in form of two-dimensional layer or dots. Furthermore, the integrity of epitaxial growth at low temperatures in terms of oxygen contamination was presented. A discussion of how sensitive the conventional characterization tools are compared to noise measurements for detection of defects has also been included. ExperimentalThe SiGe/Si multilayer structures were deposited on Si(100) substrates in temperature range of 350-650• C at 20 torr using reduced pressure chemical vapor deposition (RPCVD). The reactant gases for SiGe growth were GeH 4 or Ge 2 H 6 and SiH 4 or Si 2 H 6 as Ge and Si sources, respectively. All SiGe layers were grown below the critical thickness according to the published data.15-17 The quality of epi-layers and SiGe/Si interface could be monitored by exposing to oxygen during epitaxy or at the interfaces. For these experiments, the oxygen partial pressure was in range of 2-1600 nTorr. In order to determine the flux and absorption of oxygen in the reactor a residual gas analyzer (RGA) was c...

Multiple SiGe/Si layers epitaxy and SiGe selective etching for vertically stacked DRAM

“…The peaks of SiGe (40 nm) and (60 nm) are broadened due to dislocations. These results indicate that the thinner SiGe layers have better film quality, so the quality of Si layers grown on 20 nm SiGe is the best among the three ML structures [34,35] .…”