M.H. Xie

RHEED investigation of Ge surface segregation during gas source MBE of Si Si1 - xGex heterostructures

Abstract Ge segregation during silicon gas source molecular beam epitaxy (Si-GSMBE) has been studied by in situ growth rate measurements using reflection-high-energy-electron diffraction (RHEED) intensity oscillations. Growth rate of Si on Si 1 − x Ge x gradually decreases to the Si homoepitaxial growth rate, which is attributed to Ge surface segregation at the growth interface. This segregation has been modelled using a mass balance equation and it has been found that the observed growth rate enhancement can be used as a direct measure of the Ge segregation. Using this novel in situ technique, concentration dependence of Ge segregation was studied, and it was found that the segregation decay curve is nonlinear, resulting in two segregation regimes dependent upon the Ge concentration, consistent with previous studies. Temperature dependence studies reveal that surface hydrogen, which is produced by the dissociation of Si 2 H 6 and GeH 4 on the surface during growth, may act as a growth controlling surfactant, and comparison with solid source growth results suggests that it significantly suppresses the Ge segregation, leading to a more precise control of the interface. Finally, the thermal stability of the segregated surfaces was examined. Growth interruption and annealing during Si overlayer growth on Si 1 − x Ge x resulted in a small increase in the surface Ge concentration, which may be ascribed to the outdiffusion effect of Ge from the near surface region.

In situ observation of growth rate enhancement during gas source molecular beam epitaxy of Si1-xGex alloys on Si(100) surfaces

Using reflection‐high‐energy‐electron‐diffraction intensity oscillations the growth rate of Si1−xGex alloys at various compositions and different growth temperatures has been studied in situ. It was found that the growth rate shows a strong dependence on GeH4 flux at low temperatures (T<600 °C), while at high temperatures (T≳600 °C) the growth rate is nearly independent of the GeH4 flux but proportional to the incident Si2H6 beam flux. In addition to the enhanced growth rate, a lower activation energy is observed in the low temperature region when compared to Si homoepitaxy from Si2H6. This suggests that surface germanium atoms act as good sites for hydrogen removal which is known to inhibit Si growth from hydride sources at low temperatures. Above 600 °C, however, surface hydrogen is desorbed thermally and the addition of GeH4 has little effect on the growth rate.

Growth dynamics studied by RHEED during Si Ge epitaxy from gaseous hydrides

Abstract Si and Ge epitaxial growth from disilane and germane in a gas-source molecular beam epitaxy (GSMBE) system is followed in situ by reflection high-energy electron diffraction (RHEED) intensity oscillations. During Ge homoepitaxy, the growth rate on a Ge(100) substrate is found to be limited by surface hydrogen desorption below 350°C and by hydride adsorption above this temperature. Ge heteroepitaxy on Si results in incomplete layer growth leaving exposed Si at the surface during the initial stages of growth. Therefore, a gradual change in the observed Si surface concentration is seen as growth proceeds. Si heteroepitaxy on Ge follows the Volmer-Weber growth mode and proceeds via island formation. This, combined with Ge surface segregation, results in a slow decrease of the Ge surface population at the growth front. During heteroepitaxial growth, hydride reaction rates differ on Si and Ge surfaces, and therefore a changing concentration of the surface species is manifest as a gradual change in the observed oscillation frequency. This effect, observed during the early stages of growth, shows strong temperature dependence, consistent with previous observations on SiGe alloys. Following several layers of growth, however, the surfaces become rough. The influence of this roughness on the oscillation frequency is also discussed.

Surface segregation during molecular beam epitaxy: the site-blocking effects of surfactant atoms

The two-site exchange model for surface segregation is extended to take account of the site-blocking action of foreign atoms on the surface, either those applied deliberately as a surfactant, or those occurring as reaction products of the growth process. The model is applied to segregation of Ge during the growth of a Si film on a SiGe surface by gas-source molecular beam epitaxy, using Si2H6 as the Si precursor. It is shown how the reaction product, hydrogen, which is adsorbed on the growing surface, acts to reduce the Gibbs energy of segregation. The trends for the blocked-site concentration by hydrogen as a function of temperature and Si2H6 flux derived from the Ge segregation data show excellent agreement with reported results of surface hydrogen coverage obtained by completely different techniques.

Arsenic surface segregation and incorporation in Si and Si1-xGex during gas source molecular beam epitaxy

Abstract The surface segregation of As in Si and Si1 − xGex during gas source molecular beam epitaxy (GSMBE) has been investigated. It is shown that the segregation process is suppressed in the alloy compared with pure Si. The segregation energy is shown to be dependent on growth temperature and has been attributed to a change of surface hydrogen coverage. Surface hydrogen blocks surface sites, thus acting as a surfactant to suppress As segregation. Arsenic incorporation from AsH3 involves dissociative chemisorption via empty surface sites. The balance between the rate of dissociation, surface segregation and desorption results in a temperature and Ge concentration dependence of the net effective flux of As and consequently its concentration in the film. It has an upper limit of less than 1018 cm−3 in Si, but much higher concentrations can be achieved in SiGe alloys.

Silicon/silicon-germanium multiple quantum wells grown by gas-source molecular beam epitaxy: hydrogen coverage and interfacial abruptness

We have grown silicon-germanium/silicon (Si 1-x Ge x /Si, x < 0.30) multiple quantum wells (MQWs) by gas-source molecular beam epitaxy (GSMBE) using disilane (Si 2 H 6 ) and germane (GeH 4 ) as source gases, and have characterized their structural properties by secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD) rocking curve and transmission electron microscopy (TEM) techniques. A substrate temperature of 520°C was maintained during growth resulting in a Si and SiGe growth rate-limited primarily by hydrogen desorption kinetics. Under these conditions, surface hydrogen is expected to function as a surfactant thereby enhancing interfacial abruptness at the Si/SiGe interface through suppression of Ge surface segregation. Independent of Ge composition in the Si 1-x Ge x wells, we find abrupt interfaces, as determined from XRD measurements, and sharp SIMS decay lengths of the order of 2.5 nm/decade. For nominally identical Si barriers in all samples examined, we find thicker barriers for the structures with higher Ge content in the well. For the specimens with x = 0.30 in the wells, we find a noticeable well plus barrier period variation of approximately 5%-10% as determined from XRD rocking curves, as well as TEM evidence for the onset of strain relaxation via interface undulation formation in the first quantum well of the structure. A discussion of these results in terms of hydrogen desorption kinetics is presented.

Arsenic doping kinetics in silicon during gas source molecular beam epitaxy

Abstract Arsenic doping from arsine during Si gas source molecular beam epitaxy (GSMBE) has been investigated. The As concentration in the epitaxial film has been measured by secondary ion mass spectroscopy (SIMS) and electrochemical capacitance-voltage (eCV) analysis. It has been found to relate to the surface coverage through a segregation process, while the surface coverage itself is determined by surface adsorption/desorption kinetics, whose dependence on the AsH 3 flux indicates a non-integral-order desorption process of As from Si(100) surfaces. The surface coverage of arsenic is found to decrease the growth rate of Si from Si 2 H 6 , which can be used as a measure of As surface concentration. This enables segregation parameters such as the segregation ratio and the Gibbs energy for segregation to be extracted and they are shown to be in good agreement with previously reported results.

Growth and doping of Si and SiGe films by hydride gas-source molecular beam epitaxy

The use of molecular beams of the hydrides of Si, Ge, As and B provides an ideal vehicle for the in-situ study of the kinetics and dynamics of growth and dopant incorporation of Si and SiGe alloy films. In this article results obtained using reflection high energy electron diffraction (RHEED) and reflectance anisotropy (RA), otherwise known as reflectance difference spectroscopy (RDS), are summarised. The topics considered include basic reaction kinetics, surface segregation of Ge and As and its effect on rate processes, the influence of surface reconstruction domains on RA response and the application of gas-source molecular beam epitaxy (GSMBE) to the formation of two-dimensional electron gases in modulation-doped SiGe/Si/SiGe heterostructures.

Growth rate dependence on GeH4 during gas source MBE of SixGe 1-x alloys grown from Si2H6 and GeH4

Abstract The epitaxial growth of Si and Si x Ge 1- x alloys from molecular beams of gaseous Si (Si 2 H 6 ) and Ge (GeH 4 ) hydrides on Si(001) substrates (Si-GSMBE) has been studied using reflection high energy electron diffraction (RHEED). Diffraction patterns reveal that Ge deposited on Si results in a Stranski-Krastanow growth mode with a change in surface reconstruction in the two-dimensional (2D) regime from the two-domain Si(001)-(2×1) to a two-domain Si(2×8)-Ge, prior to the onset of three-dimensional (3D) growth. Using the RHEED intensity oscillation technique during alloy growth it is found that at substrate temperatures below 600°C there is an enhancement in growth rate of the alloy above that of pure Si growth from disilane. This phenomenon has been attributed to the increased desorption rate of hydrogen adatoms due to the presence of Ge, which leads to an acceleration of the heterogenous reaction rate. Consistent with this, during growth at substrate temperatures above the desorption rate maximum for hydrogen, 600°C no enhancement of the growth rate due to the addition of GeH 4 to the beam flux is observed and the growth rate is dependent upon disilane arrival rate. Reaction kinetics are discussed in terms of Ge concentration in the films and the ratio of GeH 4 to Si 2 H 6 in the incident beams under various growth conditions.

New hydrogen desorption kinetics from vicinal Si(0 0 1) surfaces observed by reflectance anisotropy spectroscopy

Reflectance anisotropy (RA) from vicinal Si (0 0 1) surfaces is shown to be strongly influenced by the domain structure on the (2X1) + (1X2) reconstructed surface and by adsorbates such as hydrogen. Utilizing these adsorbate effects by monitoring the time dependence of RA from single domain vicinal surfaces, an accurate determination of surface hydrogen coverage can be made. We report a zeroth order coverage dependence of hydrogen desorption which is attributed to a desorption pathway incorporating a saturated and localised precursor state in which the diffusion/migration of surface hydrogen is not the rate limiting step.