Effect of carbon on the valence band offset of Si1-x-yGexCy/Si heterojunctions

than does C-free Sil-xGex. This point is further illustrated in Fig. 2, where we calculated and plotted the equilibrium critical We have grown pseudomorphic single crystal thickness for Sil-x-yGexCy films. Therefore, by growing Sil'!Y layers on Si (100) substrates by Rapid Thermal demical x- GexCy, significant improvements can be made in the tradeVapor Deposition with up to 2.5% substitutional carbon. off between bandgap and critical thickness. Capacitance-voltage as well as admittance spectroscopy measurements have been used to study the effect of carbon on the valence band offset of compressively strained Ge C / (100) Si heterojunctions. The valence band offset of All the samples were grown by Rapid Thermal Chemical Y XY Sil-x-yGexCy/Si decreased by 25-30 meV as 1% carbon was Vapor Deposition (RTCVD). The chamber pressure was kept added. Previous studies showed that 1% carbon increased the at 6 torr. Dichlorosilane (S~~H~CIZ), germane (GeH4) and bandgap of strained Sil-xGex alloys by 21-26 meV, indicating methylsilane (SiCH6) were used as the precursors of Si, Ge, that all the change in bandgap of Sil-xGex as carbon was and C, respectively. Details of our growth system are available added is accommodated in the valence band. elsewhere[6]. The flow rates were 26 sccm for dichlorosilane, 1-4.5 sccm for germane, and 0-0.35 sccm for methylsilane, resulting in [Gel = 20%-39.5%, and [C] = 0% -2.5%. All Silx- GexCy layers were in-situ doped with diborane. P+ Sil-xThe strain in pseudomorphic Sil-xGex layers on Si substrates yC!exCy / p- Si heterostructures and p- type Si/Sil-xand resulting critical thickness imposes a severe limit on the yGe,Cy/Si structure were grown for capacitance-voltage and engineering of Si-based heterostructures. It has been shown admittance spectroscopy measurements, respectively. The that the addition carbon to form Sil-x-yGe C alloys reduces devices were formed by a single-mesa, two mask process. First the strain, with each carbon atom compensating the strain of the mesas were created by plasma etching in SF6 and then the 8-10 Ge atoms [I-21. In addition to reducing the strain, C Ti/AI metallization was patterned by lift-off. The mesa area is causes a slow increase in bandgap of SilqxGeX. 320 x 180 pm2 and the top contact area is 160 x 130 pm2. Photoluminescence (PL) measurements on Si 1 -x-yGexCy as well as transport studies of heterojunction bipolar transistor (HBT) with Sil-x-yGexCy as the base showed that the addition of 1% C increases the bandgap of Sil-xGex by 21-26 Fig. 3 shows the (400) x-ray diffraction (XRD) performed on meV[3-51. Given the bandgap increase, it implies a change of strained Sil-x-yGexCy layers with 39.5% Ge and various C valence andor conduction band offset of the Sil-xGex / (100) concentrations. The concentration of Ge was obtained by Si heterostructures when carbon is added into the Sil-xGex measuring the XRD peak relative to that of the Si substrate. layers. In this paper, we report the measurement of the valence This value is consistent with the Ge concentration obtained by band offset of Sil-x-yGexCy/Si as a function of C PL . As C is added, the peak starts shifting toward the Si peak, concentration by capacitance-voltage and admittance indicating decreased lattice constant, i.e., reduced strain. spectroscopy techniques. While C causes a slow increase in Broad peaks of Sil-x-yGexCy are indication of Scherer bandgap of Sil-xGex, we have found that reducing the strain broadening in the thin films which becomes more prominent as in Sil-xGex by adding C increases the bandgap less than does more C is added. Assuming that the Ge content was unchanged reducing the strain by merely removing Ge. Figure. 1 is the plot by the addition of methylsilane at a constant germane flow, the of bandgap versus biaxial compressive strain for both C content was quantified by measuring the relative shift of the pseudomorphic Si 1 -xGex on Si (1 00) and experimental Si 1 -x- XRD peak of Si 1 -x- GexCy layers with respect to that of Si 1 Ge C data points. As C is added to Sil-xGex and Ge xGex and use the Je:C strain compensation ratio of 8.3[2]. zont&? is held fixed, the strain decreases and bandgap Results of XRD show good single crystal quality wit increases, but the bandgap increase is much less than it would substitutional levels of up to 2.5% C. High resolution be if the strain was reduced simply by removing Ge without transmission electron microscopy (HRTEM) images of the adding C. That is, for a given bandgap, Sil-x-yGexCy has less sample with 1.2% C show good interface quality and no misfit strain and therefore allows a greater critical thickness evidence of dislocations or Sic precipitates. Fig. 4 shows the Ge C