Ultraclean Low-pressure CVD Research Articles

(Invited) Atomically Controlled Processing for Si and Ge CVD Epitaxial Growth

In this work, the segregation of dopants and surface roughness generation observed during in-situ doping and their suppression by applying an atomic layer doping approach are presented. The impurity segregations are explained by the Langmuir-type adsorption and reaction scheme. Results of nm-order thick Ge epitaxial growth on Si(100) using an ultraclean low-pressure CVD system are obtained. Degradation for epitaxial growth of Si and Ge is induced by the existence of Ge oxide in the CVD reactor. It is suggested that the evaporated Ge oxide is adsorbed on the Si surface and the Ge oxide is reduced to the pure Ge by the Si in accordance with Si oxide formation and that the Ge oxide evaporation is suppressed by SiH4 treatment. These results demonstrate the capability of CVD technology for atomically controlled processing of Si, Si1-xGex and Ge for ultra large scale integration.

(Invited) Atomically Controlled Processing for Si and Ge CVD Epitaxial Growth

Atomically controlled processing has become indispensable for the fabrication of Si-based ultrasmall devices and heterodevices for ultralarge scale integration. Our concept of atomically controlled processing is based on atomic-order surface reaction control [1-3]. The final goal is the generalization of atomic-order surface reaction processes and the creation of new properties in Si-based ultimate small structures. In the stage of nanometer-scale device fabrication, preparing atomic-order controlled surfaces and interfaces and the introduction of Ge into the Si process are required. In this work, we describe thesegregation of dopants to the surface and surface roughness generation observed during in-situ doping and their suppression by applying an atomic layer doping approach. Furthermore results of nm-order thick Ge epitaxial growth on Si(100) using an ultraclean low-pressure CVD system are presented. Figure 1 shows the P depth profile in a Si epitaxial film. Even after switching off the PH3 gas supply, P atoms are incorporated into the Si layer during Si epitaxial growth. In the steady state, it is estimated that P concentration saturates at around 3x1019cm-3 and P coverage on the Si surface is about 0.1 monolayer (6.8x1013cm-2). These values are calculated from the PH3 partial pressure and Si growth rate shown in Fig. 1 based on the surface reaction model shown in Ref. 3. The P incorporation rate, that is the product of the Si growth rate (0.8nm/min) and P concentration (about 1x1019cm-3), is about 8x1011cm-2/min. If it is assumed that the desorption of P atoms on the Si surface can be neglected [4], 6.8x1013cm-2 P atoms on the surface are incorporated into the Si epitaxial layer with 8x1011cm-2/min even after the PH3 supply has been stopped. The increase of surface roughness from RMS 0.1nm to 0.8nm during in-situ doping can be explained by the suppression of the SiH4reaction caused by the P atoms which are present on the surface. Suppression of P segregation and surface roughening are achieved using P atomic layer doping in Si. This includes the optimization of the capping Si layer deposition conditions and the P atomic amount [5] as shown in Fig. 2. This may result from low temperature Si epitaxial growth on P atoms with high growth rate. That could be demonstrated for P atomic layer doping during Ge epitaxial growth too [6]. In a second part of the paper results for nm-order-thick Ge high quality epitaxial growth on Si (100) are described. Because pure Ge and Si oxide are formed on Si substrate even at 300oC by the reaction between vaporized Ge oxide and Si substrate [7], the suppression of Ge oxide vaporization in the CVD reactor is very important. It was proposed that Ge island growth in an initial growth stage is caused by the suppression of adsorption and/or decomposition of GeH4 on the H-terminated Si surface [8]. Even on the initially H-free Si surface, Si atoms are H-terminated by the decomposition of GeH4. In order to decrease the hydrogen desorption temperature from the Si site, nm-thick Si1-xGex buffer layer on Si (100) was proposed [9,10]. The suppression of surface roughening was improved by nm-order-thick Ge epitaxial growth on nm-order-thick Si1-xGex/Si(100). The surface segregation of Si through nm-order-thick Ge layer from the substrate was observed at Ge growth temperature of 350oC. These results demonstrate the capability of atomically controlled processing for nm-order thick Si and Ge CVD epitaxial growth for ultralarge scale integration. This work is partially supported by the JSPS Core-to-Core Program, “International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration”.

(Invited) Atomic-Order Thermal Nitridation of Si, Si1-xGex and Ge by NH3

Atomically controlled processing has become indis-pensable for the fabrication of Si-based ultrasmall devices and heterodevices for ultralarge scale integration. Our concept of atomically controlled processing is based on atomic-order surface reaction control [1-3]. The final goal is the generalization of atomic-order surface reaction processes and the creation of new properties in Si-based ultimate small structures which will lead to nanometer scale Si devices as well as Si-based quantum devices. Introduction of N atomic layer at the interface between the high-k dielectric and the channel has been employed to suppress the interface trap formation [4]. Additionally, the insertion of N atomic layer within the channel may be beneficial for high mobility channel formation [5,3]. In this work, we describe atomic-order thermal nitridation of Si1-xGex (100) (x=0 - 1) Si(100) in an NH3environment in a low temperature range of 300-650°C using an ultraclean low-pressure CVD system and furthermore the thermal stability of nitride surface are discussed.It has been found that silicon nitride films are formed even at 300-650°C on Si(100) [6]. At 500°C or higher, N initially increases and tends to saturate to a certain value (N atomic amount (nN) of about 2.7 x 1015cm-2; 4 atomic layers). At 400°C or lower, on the H-terminated Si surface after wet cleaning, the nN increases and the Si-hydride coverage decreases with increasing NH3 exposure time. The Si-hydride becomes hardly observable when nN reaches the surface Si atomic amount (6.8 x 1014 cm-2). On the H-free Si surface after preheating in Ar at 650°C, nN increases up to 2 x 1014 cm-2 (Fig.1) with the appearance of the Si-hydride instantly after NH3 exposure at 300 and 400°C, indicating that NH3 molecules dissociatively are adsorbed on the Si dangling bonds. Further ultrathin nitridation is accompanied by a decrease of the Si-hydride coverage. It is found that nN is well described by Langmuir-type physical adsorption and reaction of NH3 on the Si surface [3] based on a fitting assuming that the NH3 adsorption equilibrium constant with flash heating is the same as that without flash heating [7] (Fig.2).For 30 min exposure of NH3 at partial pressure of 550 Pa and 400°C, Si1-xGex (100) and Ge (100) are also nitrided at atomic level [8-9]. After Ar or H2 heat treatment at 400-700 °C under 60 Pa, N atomic amount on the atomic-order nitrided Si (100) and Si0.6Ge0.4 (100) is scarcely changed. On the atomic-order nitrided Ge (100), N atomic amount tends to decrease with increasing the heat-treatment temperature and even by H2 purging at the cooling period after the nitridation. On the nitrided Si0.5Ge0.5 (100), amount of the nitrided Si atoms increases for the higher heat-treatment temperature, although the amount of the nitrided Ge atoms decreases (Fig.3). It is suggested that N atoms bound to Ge atoms tend to be transferred to Si atoms at temperatures above 400 °C. It is confirmed by angle-resolved XPS that preferential nitridation of Si atoms at surface over Ge atoms induces Ge segregation beneath the surface nitrided layer. Additionally, N atoms for the nitrided Si0.3Ge0.7 (100) dominantly form a Si3N4 structure which stably remains even during heat treatment in H2at 400 °C [10].

Heavy atomic-layer doping of nitrogen in Si 1 − xGe x film epitaxially grown on Si(100) by ultraclean low-pressure CVD

N atomic-layer doping in a nanometer-order Si/Si 1 − x Ge x /Si(100) heterostructure using ultraclean low-pressure chemical vapor deposition and its thermal stability at 650 °C were investigated. In the Si 0.5Ge 0.5 epitaxial layer, it is found that a N doping dose of 6 × 10 14 cm − 2 can be confined within an about 1.5 nm-thick region even after 650 °C heat treatment in contrast to the result for Si cap layer growth on the thermally nitrided Si(100) with a N doping dose of 6 × 10 14 cm − 2 which was found to be amorphous. Moreover, it is suggested that the confined N atoms in Si 1 − x Ge x preferentially form Si–N bonds and that formation of Si 3N 4 is enhanced by the heat treatment at 650 °C.

Heavy B atomic-layer doping characteristics in Si epitaxial growth on B adsorbed Si(1 0 0) by ultraclean low-pressure CVD system

Heavy B atomic-layer doping is performed by B atomic-layer formation on Si(1 0 0) using B 2H 6 and subsequent Si capping layer deposition using SiH 4. By B 2H 6 exposure on Si(1 0 0) at 180 °C, the B atom amount tends to saturate self-limitedly at around 1.4 × 10 15 cm −2 (two atomic layers (AL)). In the case of exposure at 500 °C, the B atom amount increases with time and exceeds two AL. Additionally, B oxide formation by air exposure is observed for 500 °C but not for 180 °C. Therefore, it is suggested that the H adsorption on the B AL is kept at 180 °C and H desorption from the B AL proceeds at 500 °C. It is also found that the SiH 4 exposure on the B AL at low temperatures such as 180–300 °C effectively improves the electrical activity of B and suppresses lattice strain generation even with the subsequent Si capping at 500 °C. These results demonstrate that the low temperature reaction of SiH 4 as well as B 2H 6 open the way to extremely heavy B atomic-layer doping.

Si epitaxial growth on self-limitedly B adsorbed Si 1 − xGe x(100) by ultraclean low-pressure CVD system

B atomic-layer doping in Si/Si 1 − x Ge x (100) ( x = 0.3, 1) heterostructure utilizing BCl 3 reaction on Si 1 − x Ge x (100) and subsequent Si epitaxial growth by SiH 4 reaction was investigated by ultraclean low-pressure chemical vapor deposition. On 1/3–1/2 atomic layer (2.3–3.5 × 10 14 cm − 2 ) B formed Si 1 − x Ge x (100) achieved self-limitedly with high Cl coverage, the adsorbed Cl atoms are effectively removed by SiH 4 exposure at 300 °C after BCl 3 exposure. Furthermore, it was found that the SiH 4 reaction at 300 °C on Ge(100) is enhanced by B adsorption.

Self-limited growth of Si on B atomic-layer formed Ge(1 0 0) by ultraclean low-pressure CVD system

Utilizing BCl 3 reaction on Ge(1 0 0) and subsequent Si epitaxial growth by SiH 4 reaction at 300 °C, B atomic-layer doping in Si/Ge(1 0 0) heterostructure was investigated. Cl atoms on the B atomic-layer formed Ge(1 0 0) scarcely affect upon the SiH 4 reaction. It is also found that Si atom amount deposited by SiH 4 reaction on Ge(1 0 0) is effectively enhanced by the existence of B atomic layer and the deposition rate tends to decrease at around 2–3 atomic layers which is three times larger than that in the case without B. The results of angle-resolved X-ray photoelectron spectroscopy show that most B atoms are incorporated at the heterointerface between the Si and Ge.

Epitaxial growth of P atomic layer doped Si film by alternate surface reactions of PH3 and Si2H6 on strained Si1−xGex/Si(1 0 0) in ultraclean low-pressure CVD

P atomic layer formation on Si1−xGex(1 0 0) using PH3 and subsequent Si epitaxial growth by ultraclean low-pressure CVD using Si2H6 have been investigated. The quantity of P atoms on Si1−xGex(1 0 0) tends to increase and saturate with temperature, depending on Ge fraction in the range of 200–450 °C. By Si epitaxial growth on the P surface at 450 °C using Si2H6 instead of SiH4, heavy P doping with the P concentration over 3 × 1021 cm−3 is achieved in an ultrathin region at the heterointerface with the thickness within 2 nm. For the P-doped samples, the Hall mobility of the electron is higher than that for uniformly P-doped Si with a lower P concentration of 1020 cm−3. From the mobility degradation after heat treatment up to 800 °C, it is expected that two-dimensional arrangement of P atoms is achieved for the as-deposited films.

Work function of impurity-doped polycrystalline Si1−x−yGexCy film deposited by ultraclean low-pressure CVD

Work function of in situ heavily P- or B-doped poly-Si1−x−yGexCy films were investigated. The poly-Si1−x−yGexCy films were deposited on thermally oxidized Si(100) at 550°C using an ultraclean hot-wall low-pressure chemical vapor deposition system in a SiH4–GeH4–SiH3CH3–dopnat (PH3 or B2H6) gas mixture, and the gate-to-substrate work function difference was estimated from flat-band voltage determined by high-frequency capacitance–voltage measurement. With increasing Ge and C fractions, the work function of B-doped poly-Si1−x−yGexCy film decreases, although that of P-doped poly-Si1−x−yGexCy scarcely changes. X-ray diffraction results show that the lattice constant increases with the C fraction, in other words, C is incorporated at interstitial site in the film at least at 550°C. It is suggested that the change in the work function of doped poly-Si1−x−yGexCy is universally explained by the change in the lattice constant as function of the Ge and C fractions.

Observation of sharp current peaks in resonant tunneling diode with strained Si 0.6Ge 0.4/Si(1 0 0) grown by low-temperature low-pressure CVD

p-Type double barrier resonant tunneling diodes with high-quality strained Si 0.6Ge 0.4/Si heterostructures have been realized at low temperatures using the ultraclean low-pressure CVD system. In order to suppress a voltage drop due to series resistance, the diode structure has been improved with a B-doping concentration in the p + contact layers of 10 19–10 20 cm −3. The sharp resonant current peaks have been observed from the I– V characteristics in the temperature range of 10–200 K. The peak-to-valley current ratio is as high as 2.9 at 120 K. The peak voltage decreased with increasing temperature and hovered around extremely low values with a minimum value of 8 mV at 200 K. The decrease of the peak voltage can be explained mainly by the decrease of the resonant state energy of heavy holes in the quantum well due to the increase of the hole effective mass, and by the increase of the peak energy of the hole distribution in the emitter. A sudden drop of the peak voltage was found at 110–120 K, and possible processes affecting the observed tunneling characteristics are discussed.

Layer-by-layer growth of silicon nitride films by NH3 and SiH4

Layer-by-layer growth of silicon nitride by NH 3 and SiH 4 was investigated using an ultraclean low-pressure CVD system with a Xe flash lamp. Thermal nitridation on Si(100) in an NH 3 environment with and without the flash lamp light irradiation is explained assuming Langmuir-type physical adsorption of NH 3 and reaction of the adsorbed NH 3 . The reaction can be enhanced by Xe flash lamp light irradiation, and the N atom concentration tends to saturate to ∼2.7×10 15 cm -2 . In Si deposition on the ultrathin silicon nitride, it is found that N desorption from the Si nitride films hardly occurs, and the deposited Si atom concentration increases up to about the single atomic layer concentration (6.8×10 14 cm -2 ), and then the Si deposition rate drastically increases. From FTIR/ATR and RHEED measurements, the structures of the Si film on Si and the Si nitride are found to be different. Layer-by-layer growth control of Si nitride is proposed by combining atomic-layer nitridation on Si and atomic-layer growth of Si on the Si nitride.

Control of composition and deposition rate in Si-Ge CVD epitaxy on Si

Epitaxial growth of Si-Ge on Si deposition has been investigated by ultraclean low-pressure CVD using SiH 4 and GeH 4 gases. It was found that, with GeH 4 addition, the deposition rate on Si increases, and the deposition rate strongly depends on the Si substrate orientation. From the relationship between the deposition conditions and the Ge concentration, it has been found that the SiH 4 and GeH 4 reaction rates were expressed by an equation similar to the Langmuir isotherm. Then, it is proposed that the SiH 4 reaction rate constant at Ge bond sites is larger than those of Si, while the SiH 4 and GeH 4 adsorption rate constants are largest at hollow bridge sites, which are composed of opposite bonds from Si and Ge. Moreover, an expression was derived relating the Ge content in the film to the GeH 4 and SiH 4 partial pressures.