- Email: [email protected]
Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane
TSF-34688; No of Pages 5 Thin Solid Films xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane A. Abedin ⁎, M. Moeen, C. Cappetta, M. Östling, H.H. Radamson ⁎ KTH Royal Institute of Technology, School of Information and Communication Technology, Electrum 229, 16640 Kista, Sweden
a r t i c l e
i n f o
Article history: Received 9 July 2015 Received in revised form 1 October 2015 Accepted 1 October 2015 Available online xxxx Keywords: Reduced-pressure chemical vapor deposition Low temperature epitaxy Silicon germanium Trisilane Oxygen contamination Noise measurement
a b s t r a c t This work investigates the crystal quality of SiGe layers grown at low temperatures using trisilane, and germane precursors. The crystal quality sensitivity was monitored for hydrogen chloride and/or minor oxygen amount during SiGe epitaxy or at the interface of SiGe/Si layers. The quality of the epi-layers was examined by quantifying noise parameter, K1/f obtained from the power spectral density vs. 1/f curves. The results indicate that while it is difficult to detect small defect densities in SiGe layers by physical material characterization, the noise measurement could reveal the effects of oxygen contamination as low as 0.16 mPa inside and in the interface of the layers. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Low-temperature epitaxial growth of Si epilayers and SiGe alloy layers and using them as source, drain and channel materials is a key process step for future complementary metal oxide semiconductor (CMOS) technologies [1–4]. As these group-IV layers are fully strained and highly doped, low thermal budget and high growth rate are required to avoid any strain relaxation and undesirable dopant out diffusion. In epitaxy, the thermal budget is defined by both pre-epi treatment temperature as well as the growth temperature. Classical Si precursors such as silane (SiH4) and dichlorosilane (SiH2Cl2) are used for epitaxial growth of Si and Si1 − xGex layers [5]. These sources are not sufficiently reactive at temperatures below 550 °C, and the growth rate is extremely reduced in lower temperatures [6]. One of the possible routes for reducing the growth temperature while keeping the growth rate in a sufficient range is using higher order silane precursors such as trisilane (Si3H8) [3,7,8]. The bonding energy of Si–Si (ESi–Si = 226 KJ mol−1) is lower than bonding energy of Si–H (ESi–H = 318 KJ mol−1), thereby the activation energy of trisilane is lower than silane while it has relatively high sticking coefficient [9]. However, it is not easy to use low growth temperatures with maintained high epitaxial quality. The main concern is the high concentration of oxygen and water vapor at low growth temperatures [10]. The main source of these contaminations originates from the load-locks and the purity of the process gasses. ⁎ Corresponding authors. E-mail addresses: [email protected] (A. Abedin), [email protected] (H.H. Radamson).
This work focuses on the investigation of the crystal quality of Si1 − xGex layers grown at low temperatures using trisilane, germane and hydrogen chloride. The effects of oxygen contamination at the SiGe/Si interface or inside the SiGe layers were investigated by material and electrical characterization. The effect of HCl on Ge content and layer quality is also investigated. 2. Experiments Epitaxial Si1 − xGex layers were grown in a conventional reduced pressure chemical vapor deposition system on Si (001) substrates. The layers were grown in 2.7 kPa pressure at temperatures from 450 to 600 °C using trisilane (Si3H8), germane (GeH4), and hydrogen chloride (HCl). An oxygen source of 13.3 Pa diluted in hydrogen was used to introduce oxygen contamination inside or at the interface or SiGe/Si epitaxial layers. Trisilane is in the liquid phase at room temperature and special equipment has been used in order to evaporate it and have a stable material flux. The carrier gas was purified H2 with constant flow of 20 standard liters per minute. The thickness and Ge content of the grown SiGe layers were measured by high resolution x-ray diffraction (HRXRD) [11]. In this work, a Philips X'pert diffractometer was used which was equipped with a copper target and the acceleration voltage and current were 45 kV and 40 kA, respectively. Scanning electron microscopy (SEM) Zeiss ultra-high resolution FESEM operating at 2 kV was used for surface morphology analysis. The residual oxygen and humidity level inside the chamber was monitored by a quadrupole residual gas analyzer which
http://dx.doi.org/10.1016/j.tsf.2015.10.001 0040-6090/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
2
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
was installed in the exhaust gas line. The oxygen, Fe, Cr and Ni concentration versus depth profiles were measured by secondary ion mass spectrometry (SIMS) with a Cameca IMS 7f microanalyzer. In case of oxygen measurement, a primary beam of 15 keV Cs+ ions with 100 nA intensity was rastered over a surface area of 150–150 μm2 and I case of Fe, Cr and Ni measurement a beam of 10 keV O+ 2 was used. The intensityconcentration was calibrated using ion implanted calibration samples as reference. The sputtering time was converted to sample depth by measurement of the crater depth using a Dektak 8 stylus profilometer. The effect of oxygen contamination was studied for samples exposed to oxygen amounts from 0.16 mPa to 130 mPa inside the SiGe/Si layers or at their interface. For noise measurement devices, two quantum wells of SiGe separated by a Si spacer were sandwiched between two highly boron-doped Si layers. Si spacer layers were grown between the contacts and SiGe/Si stack to avoid the strain relaxation during silicide formation [12]. The samples were processed to fabricate pixel arrays with different sizes. The whole pixel body was passivated by SiO2. The top and the bottom contact areas were opened and Ni silicide was formed at 500 °C prior to the metallization step. The purpose of formed NiSi layers was to reduce the contact resistances [13]. Two metal layers of TiW/Al were evaporated on the samples followed by lithography and dry etching for metallization and forming gas annealing was performed at 400 °C for 30 min. Fig. 1-a and b shows a schematic of the fabricated device and a SEM cross section image of the epitaxial layers. The epitaxial quality of the fabricated structures was studied electrically by noise measurements. These measurements were performed using a battery powered resistive network, a low-noise voltage preamplifier and a HP49810 vector spectrum analyzer. Standard network theory was implemented to estimate the noise partitioning i.e. the relative amount of noise power delivered to the amplifier input (high impedance) and the bias circuit. Devices were biased with a fixed voltage of 1.2 V and the power spectrum density of the noise vs. frequency curves was carefully analyzed to obtain and compare the noise level and K1/f for all the samples.
Fig. 2. Growth rate versus temperature for epitaxy of Si using trisilane precursor.
investigate the effect of HCl, SiGe layers grown with different HCl partial pressures were analyzed. Fig. 3 shows the growth rate and Ge content trends in the layers grown at two temperatures of 600 °C and 500 °C with different HCl partial pressures. In principle, the Ge content is increased and the growth rate is decreased by introducing HCl. The reason is that HCl etches preferably Si atoms compared to Ge atoms and it is possible to control the Ge content by changing HCl flow. If the HCl amount during epitaxy increases more than a critical value (determined by the epitaxy growth parameters) the Ge content begins to decrease. This behavior can be explained by the high HCl amount that results in etching not only of Si but also of Ge atoms. As the growth rate at 500 °C with high HCl amounts was very low, the critical value was derived only for growth temperature of 600 °C. 3.2. The effect of oxygen on crystal quality of SiGe layers
3. Results and discussions 3.1. HCl effect during trisilane- & germane-based SiGe layers Si films were epitaxially grown at different temperatures from 450 °C to 600 °C and the activation energy was calculated. The results presented in Fig. 2 show that the activation energy of the trisilane growth process is 0,7 eV. The low activation energy of trisilane compared to disilane [14] and silane [9] opens opportunities for low temperature growth. Trisilane and germane are the gas precursors used for epitaxy of SiGe layers. Due to the high growth rate, low growth temperature is used and the Ge content was adjusted by adding HCl during epitaxy. In order to
The crystal quality was examined by full-width-half-maximum (FWHM) of rocking curves by HRXRD measurements which are shown in Fig. 4. The results show that by increasing the oxygen quantity up to 4.3 mPa, the SiGe peak's position and the FWHM are not affected, which indicates that there was no strain relaxation in the epitaxial layers. The sharp SiGe peak and the thickness fringes besides prove the smooth surface and high quality interface of Si and SiGe layers. In order to investigate the oxide island formation in epitaxial growth of SiGe layers, Si surface was exposed to higher oxygen partial pressures from 13.3 to 133 mPa at different temperatures up to 700 °C. SEM image shown in Fig. 5-a indicates that no oxide island was formed on the surface of the sample subjected to 13.3 mPa oxygen at exposure
Fig. 1. a) A schematic of fabricated mesas of SiGe/Si multilayer structure for noise measurements, b) cross sectional SEM image.
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
3
Fig. 3. a) Growth rate and b) Ge content of SiGe layers grown at 500 and 600 °C versus different HCl partial pressures.
temperature of 600 °C. By either increasing the oxygen level to 133 mPa (Fig. 5-b) or increasing the exposure temperature to 700 °C (Fig. 5-c) the island formation became clearly visible in SEM topography images. The oxide island formation begins with nucleation in random areas. When the surface is exposed to higher amounts of oxygen, the number of nucleation sites increases but the islands' size does not grow. On the other hand, increasing the temperature resulted in larger oxide islands with lower density. By increasing the exposure time, more oxygen atoms react with the surface and the islands grow larger. A previous study has demonstrated that when the oxide islands grow and reach a critical size of 0.02 μm2, the crystal quality of the epitaxial layer will be degraded [5]. Two samples of SiGe/Si multi layers were grown to investigate the effects of oxygen's partial pressure and the exposure temperature on its incorporation at the interface. One of the samples was exposed to four different oxygen quantities from 0.27 mPa to 2.1 mPa at 600 °C and the other one was exposed to fixed oxygen content of 1.1 mPa at three different temperatures from 400 to 600 °C. SIMS spectra of the samples are shown in Fig. 6-a and b. The results show higher amount of oxygen is incorporated at the interface of the epitaxial layers when the exposure temperature and partial pressure of oxygen is increased. The higher oxygen content for 0.27 mPa compared to 0.53 mPa exposure is due to residual oxygen atoms on Si surface. 3.3. Electrical measurements to determine the crystal quality of SiGe All electrical measurements were done in a Cascade 11000 shielded probe station with temperature controlled chuck. The sample surface temperature was also double checked by laser interferometry. Noise
Fig. 4. HRXRD patterns of SiGe layers with different oxygen exposures before epitaxial growth.
spectra were measured using a battery powered resistive network, a low-noise voltage preamplifier at a gain setting of 1000 and a HP49810 vector spectrum analyzer. Standard network theory was used to estimate the noise partitioning i.e. the relative amount of noise power delivered to the amplifier input (high impedance) and the bias circuit. A fixed voltage of 1.2 V, set by tuning the bias network, was applied during noise characterization. In this case the sample current and noise are related as shown in Eqs. (1) and (2). SI ¼
k1= f Isample β f
γ
¼
k1= f V bias =Rsample f
β
γ
2 k1= f V bias 2 ;β ¼ 2 ⇒SV ¼ Rsample SI ¼ γ f
ð1Þ
ð2Þ
where SI and SV are the current and voltage noise and the k1/f is a noise constant, which is generally smaller for single crystalline materials in comparison to polycrystalline or amorphous ones. The frequency exponent γ is close to 1. Deviation from this characteristic 1/f noise behavior is generally caused by generation-recombination noise occurring due to the presence of traps in the bulk or at the oxide-passivated surface of the active device. Two sets of SiGe/Si samples were analyzed; one group with HCl and the other one without HCl and each group were contaminated with 0.16 mPa oxygen inside SiGe layers or at their interface with Si. The results were compared with the reference sample which was fabricated without oxygen contamination. It is emphasized here that the trisilane flow was adjusted from 1 to 1.4 g/h in order to maintain the same Ge content in the epi-layers grown with and without HCl. These samples were electrically characterized and the noise spectra of these structures are shown in Fig. 7. In these measurements, K1/f value was estimated from the slope of the curve in each particular case and the summary of these results is shown in Table 1. The noise level is increased equally in oxygen treated samples compared to the reference sample independent of the location of the contaminations (inside the layers or at the interface). The reason is that the noise level is sensitive to the fluctuations of carrier concentration. The presence of oxygen behaves as traps within layer and the traps are recognized as generation and recombination sites for carrier transport through the layers. When HCl is introduced during epitaxy and combined with oxygen, the noise level was increased significantly. Thereby, more analysis was performed to evaluate the HCl gas quality and to find the reason for such high noise level in SiGe samples with HCl. In this case, two SiGe samples were grown with and without HCl and were analyzed by SIMS searching for metal contamination. Fig. 8a and b shows the depth profile and concentration of the mentioned metals in the epi-layers. The SiGe which is grown in the presence of HCl contains Cr and Fe (Fig. 8-b) meanwhile the reference samples
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
4
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
Fig. 5. Formed oxide islands on Si with oxygen exposure of a) 13.3 mPa at 600 °C, b) 133 mPa at 600 °C and c) 13.3 mPa at 700 °C.
Fig. 6. SIMS spectra of oxygen treated Si multilayers at a) different oxygen partial pressures, and b) with 1.1 mPa oxygen level but at different process temperatures.
(Fig. 8-a) are pure from such contaminations. It is worth mentioning here that the purity of HCl gas used in this study is 5.0 and the epi reactor is equipped with an HCl purifier which removes humidity. Therefore, the only source of these metal contaminations should be the impurities originated by interaction of HCl gas with the gas pipeline.
From this study we found it necessary to regularly check metal contaminations in the epi-layers since HCl is a corrosive gas and may interact with the pipe-line materials. One solution to reduce the contamination problem is evacuating the HCl line as a routine work when the reactor is not in use. 4. Conclusions In this study, the sensitivity of crystal quality of Si1 − xGex (0.13 ≤ x ≤ 0.24) layers to oxygen contamination (0.16–133 mPa) for growth temperatures of 450–600 °C using trisilane and germane was presented. Increasing HCl partial pressure during epitaxial growth of SiGe layers resulted in higher Ge content while it lowered the growth rate. Both material and electrical characterizations were performed to study the effect of oxygen at SiGe/Si interface or inside the layers grown with and without HCl. The oxide islands were observed in samples exposed to oxygen levels above 13.3 mPa. The island formation began with nucleation, and the density of the nucleation sites depended on the partial pressure of oxygen and the exposure temperature. SIMS results confirm the presence of oxygen contaminations at the interface of the layers exposed to oxygen Table 1 Summarized K1/f values from the samples shown in Fig. 7.
Fig. 7. Noise spectra of SiGe samples exposed to oxygen and noise spectra of SiGe samples both HCl treated and exposed to oxygen.
Sample
Reference
O2 prior
O2 during
HCl + O2 prior
HCl + O2 during
K1/f
2 × 10−17
2 × 10−16
3 × 10−16
8 × 10−14
3 × 10−13
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
5
Fig. 8. SIMS spectra for Fe and Cr for SiGe layers grown a) without HCl and b) with HCl.
partial pressures as low as 0.27 mPa. However, x-ray rocking curve and the FWHM analysis were not very sensitive to minor crystal defects in SiGe layers. On the contrary, noise measurements revealed the influence of minor oxygen level as 0.16 mPa on the epi-layers' quality. The noise level is increased sharply in the presence of minor oxygen level. The combination of HCl and oxygen during the epitaxial growth of SiGe layers resulted in three orders of magnitude higher noise levels. One of the reasons for such a high noise level in these samples was found to be metal contamination which usually forms in HCl pipe-line. This study recommends a regular check of quality of HCl gas as well as purity of the reactant gasses to control and lower the oxygen level and metal contaminations. Acknowledgment This project was funded by Semiconductor Research Corporation (SRC) with Texas Instruments in Dallas USA. The faculty support from KTH is greatly acknowledged.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
References [12] [1] H.H. Radamson, M. Noroozi, A. Jamshidi, P.E. Thompson, M. Östling, Strain engineering in GeSnSi materials, ECS Trans. 50 (2013) 527–531, http://dx.doi.org/10.1149/ 05009.0527ecst. [2] T.N. Adam, S. Bedell, A. Reznicek, D.K. Sadana, A. Venkateshan, T. Tsunoda, et al., Low-temperature growth of epitaxial (100) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor, J. Cryst. Growth 312 (2010) 3473–3478, http://dx.doi.org/10.1016/j.jcrysgro.2010.09.012. [3] A. Gouyé, O. Kermarrec, A. Halimaoui, Y. Campidelli, D. Rouchon, M. Burdin, et al., Low-temperature RPCVD of Si, SiGe alloy, and Si1 − yCy films on Si substrates
[13]
[14]
using trisilane (Silcore®), J. Cryst. Growth 311 (2009) 3522–3527, http://dx.doi. org/10.1016/j.jcrysgro.2009.04.011. A. Joseph, D. Coolbaugh, M. Zierak, R. Wuthrich, P. Geiss, Z. He, et al., A 0.18 μm BiCMOS technology featuring 120/100 GHz (fT/fmax) HBT and ASIC-compatible CMOS using copper interconnect, Proc. 2001 BIPOLAR/BiCMOS Circuits, Technol. Meet. (2001) 143–146 Cat. No.01CH37212. 10.1109/BIPOL.2001.957877. M. Moeen, A. Salemi, M. Kolahdouz, M. Östling, H.H. Radamson, Characterization of SiGe/Si multi-quantum wells for infrared sensing, Appl. Phys. Lett. 103 (2013), 251609http://dx.doi.org/10.1063/1.4855595. M. Bauer, S.G. Thomas, Novel chemical precursors and novel CVD strategies enabling low temperature epitaxy of Si and Si:C alloys, Thin Solid Films 518 (2010) S200–S203, http://dx.doi.org/10.1016/j.tsf.2009.10.088. S. Takeuchi, N.D. Nguyen, J. Goosens, M. Caymax, R. Loo, Si1 − xGex growth using Si3H8 by low temperature chemical vapor deposition, Thin Solid Films 518 (2010) 18–22, http://dx.doi.org/10.1016/j.tsf.2009.10.047. B. Vincent, R. Loo, W. Vandervorst, G. Brammertz, M. Caymax, Low temperature Si homo-epitaxy by reduced pressure chemical vapor deposition using dichlorosilane, silane and trisilane, J. Cryst. Growth 312 (2010) 2671–2676, http://dx.doi.org/10. 1016/j.jcrysgro.2010.06.013. J.G. Martin, H.E. O'Neal, M.A. Ring, Thermal decomposition kinetics of polysilanes: disilane, trisilane, and tetrasilane, In. J. Chem. Kinet. 22 (1990) 613–631, http://dx. doi.org/10.1002/kin.550220606. A. Gruhle, G. Schmidt, H. Beneking, H. Holzbrecher, Influence of oxygen contamination during Si low pressure vapour phase epitaxy on epitaxial layer quality, J. Cryst. Growth 110 (1991) 985–988. H.H. Radamson, J. Hållstedt, Application of high-resolution x-ray diffraction for detecting defects in SiGe(C) materials, J. Phys. Condens. Matter 17 (2005) S2315–S2322, http://dx.doi.org/10.1088/0953-8984/17/22/020. O. Nur, M. Willander, H.H. Radamson, M.R. Sardela, G.V. Hansson, C.S. Petersson, et al., Strain characterization of CoSi2/n-Si0.9Ge0.1/p-Si heterostructures, Appl. Phys. Lett. 64 (1994) 440, http://dx.doi.org/10.1063/1.111122. J. Hållstedt, M. Blomqvist, P.O.Å. Persson, L. Hultman, H.H. Radamson, The effect of carbon and germanium on phase transformation of nickel on Si1 − x − yGexCy epitaxial layers, J. Appl. Phys. 95 (2004) 2397, http://dx.doi.org/10.1063/1.1645996. J.G. Martin, M.A. Ring, H.E. O'Neal, The decomposition kinetics of disilane and the heat of formation of silylene, In. J. Chem. Kinet. 19 (1987) 715–724, http://dx.doi. org/10.1002/kin.550190805.
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane A. Abedin ⁎, M. Moeen, C. Cappetta, M. Östling, H.H. Radamson ⁎ KTH Royal Institute of Technology, School of Information and Communication Technology, Electrum 229, 16640 Kista, Sweden
a r t i c l e
i n f o
Article history: Received 9 July 2015 Received in revised form 1 October 2015 Accepted 1 October 2015 Available online xxxx Keywords: Reduced-pressure chemical vapor deposition Low temperature epitaxy Silicon germanium Trisilane Oxygen contamination Noise measurement
a b s t r a c t This work investigates the crystal quality of SiGe layers grown at low temperatures using trisilane, and germane precursors. The crystal quality sensitivity was monitored for hydrogen chloride and/or minor oxygen amount during SiGe epitaxy or at the interface of SiGe/Si layers. The quality of the epi-layers was examined by quantifying noise parameter, K1/f obtained from the power spectral density vs. 1/f curves. The results indicate that while it is difficult to detect small defect densities in SiGe layers by physical material characterization, the noise measurement could reveal the effects of oxygen contamination as low as 0.16 mPa inside and in the interface of the layers. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Low-temperature epitaxial growth of Si epilayers and SiGe alloy layers and using them as source, drain and channel materials is a key process step for future complementary metal oxide semiconductor (CMOS) technologies [1–4]. As these group-IV layers are fully strained and highly doped, low thermal budget and high growth rate are required to avoid any strain relaxation and undesirable dopant out diffusion. In epitaxy, the thermal budget is defined by both pre-epi treatment temperature as well as the growth temperature. Classical Si precursors such as silane (SiH4) and dichlorosilane (SiH2Cl2) are used for epitaxial growth of Si and Si1 − xGex layers [5]. These sources are not sufficiently reactive at temperatures below 550 °C, and the growth rate is extremely reduced in lower temperatures [6]. One of the possible routes for reducing the growth temperature while keeping the growth rate in a sufficient range is using higher order silane precursors such as trisilane (Si3H8) [3,7,8]. The bonding energy of Si–Si (ESi–Si = 226 KJ mol−1) is lower than bonding energy of Si–H (ESi–H = 318 KJ mol−1), thereby the activation energy of trisilane is lower than silane while it has relatively high sticking coefficient [9]. However, it is not easy to use low growth temperatures with maintained high epitaxial quality. The main concern is the high concentration of oxygen and water vapor at low growth temperatures [10]. The main source of these contaminations originates from the load-locks and the purity of the process gasses. ⁎ Corresponding authors. E-mail addresses: [email protected] (A. Abedin), [email protected] (H.H. Radamson).
This work focuses on the investigation of the crystal quality of Si1 − xGex layers grown at low temperatures using trisilane, germane and hydrogen chloride. The effects of oxygen contamination at the SiGe/Si interface or inside the SiGe layers were investigated by material and electrical characterization. The effect of HCl on Ge content and layer quality is also investigated. 2. Experiments Epitaxial Si1 − xGex layers were grown in a conventional reduced pressure chemical vapor deposition system on Si (001) substrates. The layers were grown in 2.7 kPa pressure at temperatures from 450 to 600 °C using trisilane (Si3H8), germane (GeH4), and hydrogen chloride (HCl). An oxygen source of 13.3 Pa diluted in hydrogen was used to introduce oxygen contamination inside or at the interface or SiGe/Si epitaxial layers. Trisilane is in the liquid phase at room temperature and special equipment has been used in order to evaporate it and have a stable material flux. The carrier gas was purified H2 with constant flow of 20 standard liters per minute. The thickness and Ge content of the grown SiGe layers were measured by high resolution x-ray diffraction (HRXRD) [11]. In this work, a Philips X'pert diffractometer was used which was equipped with a copper target and the acceleration voltage and current were 45 kV and 40 kA, respectively. Scanning electron microscopy (SEM) Zeiss ultra-high resolution FESEM operating at 2 kV was used for surface morphology analysis. The residual oxygen and humidity level inside the chamber was monitored by a quadrupole residual gas analyzer which
http://dx.doi.org/10.1016/j.tsf.2015.10.001 0040-6090/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
2
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
was installed in the exhaust gas line. The oxygen, Fe, Cr and Ni concentration versus depth profiles were measured by secondary ion mass spectrometry (SIMS) with a Cameca IMS 7f microanalyzer. In case of oxygen measurement, a primary beam of 15 keV Cs+ ions with 100 nA intensity was rastered over a surface area of 150–150 μm2 and I case of Fe, Cr and Ni measurement a beam of 10 keV O+ 2 was used. The intensityconcentration was calibrated using ion implanted calibration samples as reference. The sputtering time was converted to sample depth by measurement of the crater depth using a Dektak 8 stylus profilometer. The effect of oxygen contamination was studied for samples exposed to oxygen amounts from 0.16 mPa to 130 mPa inside the SiGe/Si layers or at their interface. For noise measurement devices, two quantum wells of SiGe separated by a Si spacer were sandwiched between two highly boron-doped Si layers. Si spacer layers were grown between the contacts and SiGe/Si stack to avoid the strain relaxation during silicide formation [12]. The samples were processed to fabricate pixel arrays with different sizes. The whole pixel body was passivated by SiO2. The top and the bottom contact areas were opened and Ni silicide was formed at 500 °C prior to the metallization step. The purpose of formed NiSi layers was to reduce the contact resistances [13]. Two metal layers of TiW/Al were evaporated on the samples followed by lithography and dry etching for metallization and forming gas annealing was performed at 400 °C for 30 min. Fig. 1-a and b shows a schematic of the fabricated device and a SEM cross section image of the epitaxial layers. The epitaxial quality of the fabricated structures was studied electrically by noise measurements. These measurements were performed using a battery powered resistive network, a low-noise voltage preamplifier and a HP49810 vector spectrum analyzer. Standard network theory was implemented to estimate the noise partitioning i.e. the relative amount of noise power delivered to the amplifier input (high impedance) and the bias circuit. Devices were biased with a fixed voltage of 1.2 V and the power spectrum density of the noise vs. frequency curves was carefully analyzed to obtain and compare the noise level and K1/f for all the samples.
Fig. 2. Growth rate versus temperature for epitaxy of Si using trisilane precursor.
investigate the effect of HCl, SiGe layers grown with different HCl partial pressures were analyzed. Fig. 3 shows the growth rate and Ge content trends in the layers grown at two temperatures of 600 °C and 500 °C with different HCl partial pressures. In principle, the Ge content is increased and the growth rate is decreased by introducing HCl. The reason is that HCl etches preferably Si atoms compared to Ge atoms and it is possible to control the Ge content by changing HCl flow. If the HCl amount during epitaxy increases more than a critical value (determined by the epitaxy growth parameters) the Ge content begins to decrease. This behavior can be explained by the high HCl amount that results in etching not only of Si but also of Ge atoms. As the growth rate at 500 °C with high HCl amounts was very low, the critical value was derived only for growth temperature of 600 °C. 3.2. The effect of oxygen on crystal quality of SiGe layers
3. Results and discussions 3.1. HCl effect during trisilane- & germane-based SiGe layers Si films were epitaxially grown at different temperatures from 450 °C to 600 °C and the activation energy was calculated. The results presented in Fig. 2 show that the activation energy of the trisilane growth process is 0,7 eV. The low activation energy of trisilane compared to disilane [14] and silane [9] opens opportunities for low temperature growth. Trisilane and germane are the gas precursors used for epitaxy of SiGe layers. Due to the high growth rate, low growth temperature is used and the Ge content was adjusted by adding HCl during epitaxy. In order to
The crystal quality was examined by full-width-half-maximum (FWHM) of rocking curves by HRXRD measurements which are shown in Fig. 4. The results show that by increasing the oxygen quantity up to 4.3 mPa, the SiGe peak's position and the FWHM are not affected, which indicates that there was no strain relaxation in the epitaxial layers. The sharp SiGe peak and the thickness fringes besides prove the smooth surface and high quality interface of Si and SiGe layers. In order to investigate the oxide island formation in epitaxial growth of SiGe layers, Si surface was exposed to higher oxygen partial pressures from 13.3 to 133 mPa at different temperatures up to 700 °C. SEM image shown in Fig. 5-a indicates that no oxide island was formed on the surface of the sample subjected to 13.3 mPa oxygen at exposure
Fig. 1. a) A schematic of fabricated mesas of SiGe/Si multilayer structure for noise measurements, b) cross sectional SEM image.
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
3
Fig. 3. a) Growth rate and b) Ge content of SiGe layers grown at 500 and 600 °C versus different HCl partial pressures.
temperature of 600 °C. By either increasing the oxygen level to 133 mPa (Fig. 5-b) or increasing the exposure temperature to 700 °C (Fig. 5-c) the island formation became clearly visible in SEM topography images. The oxide island formation begins with nucleation in random areas. When the surface is exposed to higher amounts of oxygen, the number of nucleation sites increases but the islands' size does not grow. On the other hand, increasing the temperature resulted in larger oxide islands with lower density. By increasing the exposure time, more oxygen atoms react with the surface and the islands grow larger. A previous study has demonstrated that when the oxide islands grow and reach a critical size of 0.02 μm2, the crystal quality of the epitaxial layer will be degraded [5]. Two samples of SiGe/Si multi layers were grown to investigate the effects of oxygen's partial pressure and the exposure temperature on its incorporation at the interface. One of the samples was exposed to four different oxygen quantities from 0.27 mPa to 2.1 mPa at 600 °C and the other one was exposed to fixed oxygen content of 1.1 mPa at three different temperatures from 400 to 600 °C. SIMS spectra of the samples are shown in Fig. 6-a and b. The results show higher amount of oxygen is incorporated at the interface of the epitaxial layers when the exposure temperature and partial pressure of oxygen is increased. The higher oxygen content for 0.27 mPa compared to 0.53 mPa exposure is due to residual oxygen atoms on Si surface. 3.3. Electrical measurements to determine the crystal quality of SiGe All electrical measurements were done in a Cascade 11000 shielded probe station with temperature controlled chuck. The sample surface temperature was also double checked by laser interferometry. Noise
Fig. 4. HRXRD patterns of SiGe layers with different oxygen exposures before epitaxial growth.
spectra were measured using a battery powered resistive network, a low-noise voltage preamplifier at a gain setting of 1000 and a HP49810 vector spectrum analyzer. Standard network theory was used to estimate the noise partitioning i.e. the relative amount of noise power delivered to the amplifier input (high impedance) and the bias circuit. A fixed voltage of 1.2 V, set by tuning the bias network, was applied during noise characterization. In this case the sample current and noise are related as shown in Eqs. (1) and (2). SI ¼
k1= f Isample β f
γ
¼
k1= f V bias =Rsample f
β
γ
2 k1= f V bias 2 ;β ¼ 2 ⇒SV ¼ Rsample SI ¼ γ f
ð1Þ
ð2Þ
where SI and SV are the current and voltage noise and the k1/f is a noise constant, which is generally smaller for single crystalline materials in comparison to polycrystalline or amorphous ones. The frequency exponent γ is close to 1. Deviation from this characteristic 1/f noise behavior is generally caused by generation-recombination noise occurring due to the presence of traps in the bulk or at the oxide-passivated surface of the active device. Two sets of SiGe/Si samples were analyzed; one group with HCl and the other one without HCl and each group were contaminated with 0.16 mPa oxygen inside SiGe layers or at their interface with Si. The results were compared with the reference sample which was fabricated without oxygen contamination. It is emphasized here that the trisilane flow was adjusted from 1 to 1.4 g/h in order to maintain the same Ge content in the epi-layers grown with and without HCl. These samples were electrically characterized and the noise spectra of these structures are shown in Fig. 7. In these measurements, K1/f value was estimated from the slope of the curve in each particular case and the summary of these results is shown in Table 1. The noise level is increased equally in oxygen treated samples compared to the reference sample independent of the location of the contaminations (inside the layers or at the interface). The reason is that the noise level is sensitive to the fluctuations of carrier concentration. The presence of oxygen behaves as traps within layer and the traps are recognized as generation and recombination sites for carrier transport through the layers. When HCl is introduced during epitaxy and combined with oxygen, the noise level was increased significantly. Thereby, more analysis was performed to evaluate the HCl gas quality and to find the reason for such high noise level in SiGe samples with HCl. In this case, two SiGe samples were grown with and without HCl and were analyzed by SIMS searching for metal contamination. Fig. 8a and b shows the depth profile and concentration of the mentioned metals in the epi-layers. The SiGe which is grown in the presence of HCl contains Cr and Fe (Fig. 8-b) meanwhile the reference samples
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
4
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
Fig. 5. Formed oxide islands on Si with oxygen exposure of a) 13.3 mPa at 600 °C, b) 133 mPa at 600 °C and c) 13.3 mPa at 700 °C.
Fig. 6. SIMS spectra of oxygen treated Si multilayers at a) different oxygen partial pressures, and b) with 1.1 mPa oxygen level but at different process temperatures.
(Fig. 8-a) are pure from such contaminations. It is worth mentioning here that the purity of HCl gas used in this study is 5.0 and the epi reactor is equipped with an HCl purifier which removes humidity. Therefore, the only source of these metal contaminations should be the impurities originated by interaction of HCl gas with the gas pipeline.
From this study we found it necessary to regularly check metal contaminations in the epi-layers since HCl is a corrosive gas and may interact with the pipe-line materials. One solution to reduce the contamination problem is evacuating the HCl line as a routine work when the reactor is not in use. 4. Conclusions In this study, the sensitivity of crystal quality of Si1 − xGex (0.13 ≤ x ≤ 0.24) layers to oxygen contamination (0.16–133 mPa) for growth temperatures of 450–600 °C using trisilane and germane was presented. Increasing HCl partial pressure during epitaxial growth of SiGe layers resulted in higher Ge content while it lowered the growth rate. Both material and electrical characterizations were performed to study the effect of oxygen at SiGe/Si interface or inside the layers grown with and without HCl. The oxide islands were observed in samples exposed to oxygen levels above 13.3 mPa. The island formation began with nucleation, and the density of the nucleation sites depended on the partial pressure of oxygen and the exposure temperature. SIMS results confirm the presence of oxygen contaminations at the interface of the layers exposed to oxygen Table 1 Summarized K1/f values from the samples shown in Fig. 7.
Fig. 7. Noise spectra of SiGe samples exposed to oxygen and noise spectra of SiGe samples both HCl treated and exposed to oxygen.
Sample
Reference
O2 prior
O2 during
HCl + O2 prior
HCl + O2 during
K1/f
2 × 10−17
2 × 10−16
3 × 10−16
8 × 10−14
3 × 10−13
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001
A. Abedin et al. / Thin Solid Films xxx (2015) xxx–xxx
5
Fig. 8. SIMS spectra for Fe and Cr for SiGe layers grown a) without HCl and b) with HCl.
partial pressures as low as 0.27 mPa. However, x-ray rocking curve and the FWHM analysis were not very sensitive to minor crystal defects in SiGe layers. On the contrary, noise measurements revealed the influence of minor oxygen level as 0.16 mPa on the epi-layers' quality. The noise level is increased sharply in the presence of minor oxygen level. The combination of HCl and oxygen during the epitaxial growth of SiGe layers resulted in three orders of magnitude higher noise levels. One of the reasons for such a high noise level in these samples was found to be metal contamination which usually forms in HCl pipe-line. This study recommends a regular check of quality of HCl gas as well as purity of the reactant gasses to control and lower the oxygen level and metal contaminations. Acknowledgment This project was funded by Semiconductor Research Corporation (SRC) with Texas Instruments in Dallas USA. The faculty support from KTH is greatly acknowledged.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
References [12] [1] H.H. Radamson, M. Noroozi, A. Jamshidi, P.E. Thompson, M. Östling, Strain engineering in GeSnSi materials, ECS Trans. 50 (2013) 527–531, http://dx.doi.org/10.1149/ 05009.0527ecst. [2] T.N. Adam, S. Bedell, A. Reznicek, D.K. Sadana, A. Venkateshan, T. Tsunoda, et al., Low-temperature growth of epitaxial (100) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor, J. Cryst. Growth 312 (2010) 3473–3478, http://dx.doi.org/10.1016/j.jcrysgro.2010.09.012. [3] A. Gouyé, O. Kermarrec, A. Halimaoui, Y. Campidelli, D. Rouchon, M. Burdin, et al., Low-temperature RPCVD of Si, SiGe alloy, and Si1 − yCy films on Si substrates
[13]
[14]
using trisilane (Silcore®), J. Cryst. Growth 311 (2009) 3522–3527, http://dx.doi. org/10.1016/j.jcrysgro.2009.04.011. A. Joseph, D. Coolbaugh, M. Zierak, R. Wuthrich, P. Geiss, Z. He, et al., A 0.18 μm BiCMOS technology featuring 120/100 GHz (fT/fmax) HBT and ASIC-compatible CMOS using copper interconnect, Proc. 2001 BIPOLAR/BiCMOS Circuits, Technol. Meet. (2001) 143–146 Cat. No.01CH37212. 10.1109/BIPOL.2001.957877. M. Moeen, A. Salemi, M. Kolahdouz, M. Östling, H.H. Radamson, Characterization of SiGe/Si multi-quantum wells for infrared sensing, Appl. Phys. Lett. 103 (2013), 251609http://dx.doi.org/10.1063/1.4855595. M. Bauer, S.G. Thomas, Novel chemical precursors and novel CVD strategies enabling low temperature epitaxy of Si and Si:C alloys, Thin Solid Films 518 (2010) S200–S203, http://dx.doi.org/10.1016/j.tsf.2009.10.088. S. Takeuchi, N.D. Nguyen, J. Goosens, M. Caymax, R. Loo, Si1 − xGex growth using Si3H8 by low temperature chemical vapor deposition, Thin Solid Films 518 (2010) 18–22, http://dx.doi.org/10.1016/j.tsf.2009.10.047. B. Vincent, R. Loo, W. Vandervorst, G. Brammertz, M. Caymax, Low temperature Si homo-epitaxy by reduced pressure chemical vapor deposition using dichlorosilane, silane and trisilane, J. Cryst. Growth 312 (2010) 2671–2676, http://dx.doi.org/10. 1016/j.jcrysgro.2010.06.013. J.G. Martin, H.E. O'Neal, M.A. Ring, Thermal decomposition kinetics of polysilanes: disilane, trisilane, and tetrasilane, In. J. Chem. Kinet. 22 (1990) 613–631, http://dx. doi.org/10.1002/kin.550220606. A. Gruhle, G. Schmidt, H. Beneking, H. Holzbrecher, Influence of oxygen contamination during Si low pressure vapour phase epitaxy on epitaxial layer quality, J. Cryst. Growth 110 (1991) 985–988. H.H. Radamson, J. Hållstedt, Application of high-resolution x-ray diffraction for detecting defects in SiGe(C) materials, J. Phys. Condens. Matter 17 (2005) S2315–S2322, http://dx.doi.org/10.1088/0953-8984/17/22/020. O. Nur, M. Willander, H.H. Radamson, M.R. Sardela, G.V. Hansson, C.S. Petersson, et al., Strain characterization of CoSi2/n-Si0.9Ge0.1/p-Si heterostructures, Appl. Phys. Lett. 64 (1994) 440, http://dx.doi.org/10.1063/1.111122. J. Hållstedt, M. Blomqvist, P.O.Å. Persson, L. Hultman, H.H. Radamson, The effect of carbon and germanium on phase transformation of nickel on Si1 − x − yGexCy epitaxial layers, J. Appl. Phys. 95 (2004) 2397, http://dx.doi.org/10.1063/1.1645996. J.G. Martin, M.A. Ring, H.E. O'Neal, The decomposition kinetics of disilane and the heat of formation of silylene, In. J. Chem. Kinet. 19 (1987) 715–724, http://dx.doi. org/10.1002/kin.550190805.
Please cite this article as: A. Abedin, et al., Sensitivity of the crystal quality of SiGe layers grown at low temperatures by trisilane and germane, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.10.001