Ultrahigh-vacuum Chemical Vapor Deposition System Research Articles

SiyGe1−x−ySnx films grown on Si using a cold-wall ultrahigh-vacuum chemical vapor deposition system

Silicon germanium tin alloys were grown directly on Si substrates using a cold-wall ultrahigh-vacuum chemical vapor deposition system at 300 °C, where commercially available precursors of silane, germane, and stannic chloride were used to grow the epitaxial layers. The crystallinity and growth quality of the SiyGe1−x−ySnx films were investigated through material characterization methods including x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. Rutherford backscattering measurements show that 2%–5% of the Sn and 3%–5% of the Si were successfully incorporated. Investigation of the material growth parameters shows that a flow rate of stannic chloride higher than 1 sccm results in etching of the film, while an increase in the silane flow rate results in amorphous film growth. The photoluminescence study shows clear emission peaks ascribed to direct and indirect bandgap transitions, which are in agreement with theoretical calculations.

Voltage-tunable dual-band infrared photodetectors with Si/SiGe metal–semiconductor–metal heterostructure

A simple and low-cost structure of voltage-tunable dual-band near-infrared photodetector (PD) has been proposed, in which the PDs were developed by using Si 0.8Ge 0.2/Si metal–semiconductor–metal (MSM) heterostructure. The Si 0.8Ge 0.2/Si layers were deposited by ultrahigh-vacuum chemical vapor deposition system and a transparent layer of indium–tin oxide (ITO) was used as a metal layer to enhance the entrance of photons. In this study, we found that only one band was detected with a peak wavelength located at 950 nm at zero applied bias. When bias was increased to 1 V, in contrast a dual-band was achieved, where two peak wavelengths were centered at 950- and 1150-nm. It is suggested that the two bands are the absorption of top-Si and bottom-Si 0.8Ge 0.2 layers, respectively. The spectra of Si bulk and Si 0.8Ge 0.2 layer were also measured to verify our results and relating mechanisms are explained here.

Growth of Ge Layer on Relaxed Ge-Rich SiGe by Ultrahigh Vacuum Chemical Vapor Deposition

The paper describes the growth of a germanium (Ge) film on a thin relaxed Ge-rich SiGe buffer. The thin Ge-rich SiGe buffer layer was achieved through a combination of ultrahigh vacuum chemical vapor deposition (UHVCVD) SiGe epitaxial growth and SiGe oxidation. A lower Ge content strained SiGe layer was first grown on the Si (001) substrate and then the Ge mole fraction was increased by oxidation. After removal of the surface oxide, a higher Ge content SiGe layer was grown and oxidized again. The Ge mole fraction was increased to 0.8 in the 50 nm thick SiGe layer. Finally a 150 nm thick pure Ge film was grown on the SiGe buffer layer using the UHVCVD system. This technique produces a much thinner buffer than the conventional compositionally graded relaxed SiGe method with the same order of magnitude threading dislocation density.

Using thin-Al films to boost the quantum efficiency of SiGe/Si multi-quantum well avalanche photodiodes

Thin-Al films have been utilized to boost the quantum efficiency of SiGe/Si multi-quantum well avalanche photodiodes (MQW-APDs), which are fabricated by using ultrahigh-vacuum chemical vapor deposition system. In this structure, in addition to the conventional absorption layer of MQW, an undoped strained-Si 0.8Ge 0.2 layer is inserted to enhance the absorption of MQW-APDs. It is found that the MQW-APDs with a thin-Al coating have a better performance than those with thin-Al-coating ones. In the fabricated APDs, an avalanche multiplication occurred at about 2 V reverse-bias voltages. At unmultiplied voltage of 1 V, the responsivity is only 0.41 A/W. When bias is increased to 4 V, a significant avalanche multiplication is achieved and the responsivity is as high as 3.5 A/W for the APDs with thin-Al coating. As compared to the APD without thin-Al coating, the one with thin-Al coating has a higher responsivity and quantum efficiency by a magnitude of 2.1 under a 4-V reverse-bias voltage for 850-nm light source.

Low cost wavelength filter of SiGe photodetector with a-Si:H capped layer

The strained Si 0.8Ge 0.2 film has been prepared onto Si substrate by using an ultrahigh-vacuum chemical vapor deposition system. A low cost wavelength filter of photodetector has been demonstrated for the first time. This filter was simply carried out by just inserting a 60 nm thick a-Si:H capped layer onto Si 0.8Ge 0.2 thin film. The room-temperature photoluminescence shows that the sample with Si 0.8Ge 0.2 layer has a tendency to shift wavelength into longer regime than that of Si substrate. The full width at half maximum (FWHM) was 185 nm for Si 0.8Ge 0.2 photodetector without a-Si:H capped. By inserting a 60 nm thick a-Si:H capped layer, the FWHM was narrowed into 97 nm. This demonstrates that the a-Si:H capped layer has an ability acted as wavelength filter in our study.

Poly-SiGe films prepared by metal-induced growth using UHVCVD system

Poly-SiGe films were prepared by a metal-induced growth technique with ultrahigh vacuum chemical vapor deposition(UHVCVD) at low temperature. The crystal quality and morphology of poly-SiGe films were characterized by XRD and SEM. The influences of varying the thickness of Ni prelayer and the growth parameters on poly-SiGe films were investigated. It is shown that thicker Ni(≥10nm) has an obvious effect on poly-SiGe growth at 420—500℃. And for the samples with 60nm thick Ni layers, poly-SiGe deposition yields a continuous, highly crystalline film with good morphology.

Boron-δ doped Si grown by ultra-high vacuum chemical vapor deposition

We report on the properties of p-type boron-δ doped layers in Si prepared by novel ultra-high vacuum chemical vapor deposition (UHVCVD) using the growth interruption. This UHVCVD system, with a water-cooled stainless steel growth chamber, allows a higher growth rate and also maintains a high-quality film quality. To suppress the bordering of δ-doping distribution during the growth, the growth is proceeding at a low temperature (550 °C). Secondary-ion mass spectrometry (SIMS) analysis reveals that peak B concentration is linear in the doping time at a fixed B 2H 6 flow rate. Furthermore, sharp B depth profile of higher concentration was obtained after increasing of B 2H 6 flow rate and pumping time between steps. The peak concentration of doping spikes reaches to 1×10 19 cm −3 and the full-width at half-maximum (FWHM) lowers to 67 Å with the up slope as steep as 5 nm per decade.

In situ RHEED monitoring of carbon incorporation during SiGeC/Si(001) growth in a UHV-CVD system

In situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM) and high resolution X-ray diffraction (XRD) have been combined to investigate the kinetics of carbon incorporation during the growth of SiGeC alloys on Si(001) in an ultrahigh-vacuum chemical-vapor deposition system. At a given growth condition of substrate temperature and total growth pressure, three distinct growth regimes can be identified from RHEED as a function of the methylsilane partial pressure. At low methylsilane partial pressures, a two-dimensional growth RHEED pattern is observed and TEM investigations indicate that the grown films are defect-free up to a thickness of at least three times larger than the critical thickness without carbon. When the methylsilane partial pressure exceeds a threshold value, a transition from two-dimensional growth to islanding growth is detected from RHEED. At the same time, the substitutional carbon content—as determined by XRD rocking curves—is found to decrease. For high methylsilane partial pressures, RHEED patterns reveal the appearance of extra diffraction spots, which can be attributed to the formation of stacking faults and/or twins along the [111] planes. Since the growth mode transition during SiGeC growth is found to occur only after the deposition of a certain thickness—whose value depends on the methylsilane flow—we propose a concept of multilayer-array growth, following which defect-free SiGeC films with substitutional carbon contents higher than 3% can be achieved.

Multilayer-array growth of SiGeC alloys on Si(001)

The growth of SiGeC alloys on Si(001) in an ultrahigh vacuum chemical-vapor deposition system was investigated by means of in situ reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. It is shown that when the total amount of deposited carbon exceeds a value of about 1.5%, the grown layers contain a high density of stacking faults and/or microtwins. However, such defects are found to be formed only after the deposition of a certain thickness, whose value depends on the deposited carbon amount. By realizing SiGeC/Si multilayer arrays, we show that defect-free SiGeC films with a substitutional carbon content up to 3.3% can be achieved.

Photoluminescence study of initial interdiffusion of SiGe/Si quantum wells grown by ultrahigh vacuum-chemical vapor deposition

SiGe quantum wells were grown at 525 °C using a commercially available, ultrahigh vacuum–chemical vapor deposition system, in which the purity of the material and quality of interfaces have already been demonstrated. Changes in photoluminescence line energies are monitored and the extent of interdiffusion in the wells during annealing is calculated. A strong initial enhancement of the diffusivity is observed in as-grown material. Material annealed using a two-step process in which strain and Ge peak concentrations are unchanged after the first (low temperature) step, shows a much lower interdiffusion during the second step. It is argued that strain alone cannot explain the enhanced interdiffusion, which is, thus, attributed to grown-in, nonequilibrium point defects.

Uniformity of epilayer grown by ultrahigh-vacuum chemical vapor deposition

In this work, we explored the Ge fraction, layer thickness and dopant concentration uniformity of epilayers grown by an ultrahigh-vacuum chemical vapor deposition system (UHV/CVD). Three epilayers were grown for this study: a single epilayer of SiGe, a heavily boron doped Si epilayer and a Si/SiGe superlattice with p + Si cap. The uniformity in a wafer was measured to be less than ±1.5%. In addition, we explored the wafer-to-wafer uniformity of a strained SiGe layer. The variations in thickness and composition between two samples grown in the same run were evaluated to be ±1.3 and ±1.2%, respectively. These results show that uniform layers can be simultaneously obtained on many wafers by the UHV/CVD system.

Low-Temperature Epitaxial Growth of Silicon and Silicon-Germanium Alloy by Ultrahigh-Vacuum Chemical Vapor Deposition

Low-temperature epitaxy of silicon and silicon-germanium alloy via an ultrahigh-vacuum chemical vapor deposition system was investigated. Bistable conditions were observed for silicon epitaxial growth performed within the temperature range of 550° C to 800° C. The activation energy of the SiGe growth rate was found to decrease as the germanium composition increased. The germanium atomic molar fraction in these epitaxial layers was tightly controlled by a computer-controlled gas source switching system. Si/SiGe superlattice structures of 20-period 5 nm Si/12 nm SiGe layers were grown to demonstrate the excellent controllability of this growth technique.