Normal incidence p-i-n Ge heterojunction photodiodes on Si substrate grown by ultrahigh vacuum chemical vapor deposition

We report on normal incidence p-i-n heterojunction photodiodes operating in the near-infrared region and realized in pure germanium on planar silicon substrate. The diodes were fabricated by ultrahigh vacuum chemical vapor deposition at 600 °C without thermal annealing and allowing the integration with standard silicon processes. Due to the 0.14% residual tensile strain generated by the thermal expansion mismatch between Ge and Si, an efficiency enhancement of nearly 3-fold at 1.55 μm and the absorption edge shifting to longer wavelength of about 40 nm are achieved in the epitaxial Ge films. The diode with a responsivity of 0.23 A/W at 1.55 μm wavelength and a bulk dark current density of 10 mA/cm² is demonstrated. These diodes with high performances and full compatibility with the CMOS processes enable monolithically integrating microphotonics and microelectronics on the same chip.     

Formation of rippled surface morphology during Si/Si （100） epitaxy by ultrahigh vacuum chemical vapour deposition

The Si epitaxial films are grown on Si (100) substrates using pure Si2H6 as a gas source using ultrahigh vacuum chemical vapour deposition technology. The values of growth temperature T_g are 650 ℃, 700 ℃, 730 ℃, 750 ℃, and 800 ℃. Growth mode changes from island mode to step-flow mode with T_g increasing from 650 ℃ to 700 ℃. Rippled surface morphologies are observed at T_g = 700 ℃, 730 ℃, and 800 ℃, but disappear when T_g = 750 ℃. A model is presented to explain the formation and the disappearance of the ripples by considering the stability of the step-flow growth.     

Morphology of heteroepitaxial β-SiC films grown on Si(111) through high-vacuum chemical vapor deposition from hexane vapors

The characteristics of cubic silicon carbide films grown on silicon through high-vacuum chemical vapor deposition (HVCVD) from hexane vapors are investigated using scanning probe microscopy and x-ray diffraction. The surface morphology and structure of the films are analyzed as a function of the thickness of the deposited films and the nature of the substrate (silicon, sapphire). The role of different diffusion fluxes arising in the structure and the related possible mechanisms of growth of β-SiC layers on silicon substrates are discussed.     

Metalorganic chemical vapor phase epitaxy and structural properties of Ga1-xPxN on GaN/Si(111) substrates

GaPN-layers with phosphorus concentrations up to 4.4% were grown on GaN/Si(111) substrates by metal organic vapour phase epitaxy. The growth temperature and phosphine flows were varied in order to investigate the growth characteristics of the GaPN layers. The layers were investigated by X-ray diffraction, transmission electron diffraction, energy dispersive X-ray measurements, scanning electron microscopy, transmission electron microscopy, and photoluminescence. Singlecrystalline wurtzite-type epitaxial GaPN(0001) layers were obtained for x<0.05. For such layers X-ray and transmission electron diffraction measurements show a reduction of both the c- and a-latticeparameters without a change of the wurtzite-type crystal structure, thus indicating that phosphorus is incorporated as P³⁺ or P⁵⁺ likely substituting the Gallium lattice sites of the GaN-crystal, i.e. the formation of Ga_1-xP_xN and not GaN_1-xP_x as might be expected. For higher phosphorous contents phase separation effects and a variety of different phases were observed. PACS 61.43.Dq; 68.55.Jk; 81.15.Gh; 81.05.Ea     

Surface treatments toward obtaining clean GaN(0 0 0 1) from commercial hydride vapor phase epitaxy and metal-organic chemical vapor deposition substrates in ultrahigh vacuum

We studied processes of cleaning GaN(0 0 0 1) surfaces on four different types of wafers: two types were hydride vapor phase epitaxy (HVPE) free-standing substrates and two types were metal-organic chemical vapor deposition (MOCVD) films grown on these HVPE substrates and prepared by annealing and/or Ar ion sputtering in ultra high vacuum. We observed the surfaces through treatments using in situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy, and also using ex situ temperature programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction, and secondary ion mass spectrometry. For HVPE samples, we obtained relatively clean surfaces under optimized three-step annealing conditions (200 °C for 12 h + 400 °C for 1 h + 500 °C for 5 min) without sputtering, after which the surface contamination of oxide and carbide was reduced to ∼20% of that before annealing. Clear GaN(0 0 0 1)1×1 patterns were obtained by LEED and RHEED. STM images showed flat terraces of ∼10 nm size and steps of ∼0.5 nm height. Upon annealing the HVPE-GaN samples at a much higher temperature (C), three-dimensional (3D) islands with facets were formed and the surface stoichiometry was broken down with the desorption of nitrogen in the form of ammonia, since the samples include hydrogen as an impurity. Ar⁺ sputtering was effective for removing surface contamination, however, postannealing could not recover the surface roughness but promoted the formation of 3D islands on the surface. For MOCVD/HVPE homoepitaxial samples, the surfaces are terminated by hydrogen and the as-introduced samples showed a clear 1×1 structure. Upon annealing at 500-600 °C, the surface hydrogen was removed and a 3×3 reconstruction structure partially appeared, although a 1×1 structure was dominant. We summarize the structure differences among the samples under the same treatment and clarify the effect of crystal quality, such as dislocations, the concentration of hydrogen impurities, and the residual reactant molecules in GaN films, on the surface structure.     

In-situ wafer bowing measurements of GaN grown on Si(111) substrate by reflectivity mapping in metal organic chemical vapor deposition system

In this work, the wafer bowing during growth can be in-situ measured by a reflectivity mapping method in the 3×2 Thomas Swan close coupled showerhead metal organic chemical vapor deposition(MOCVD) system. The reflectivity mapping method is usually used to measure the film thickness and growth rate. The wafer bowing caused by stresses(tensile and compressive) during the epitaxial growth leads to a temperature variation at different positions on the wafer, and the lower growth temperature leads to a faster growth rate and vice versa. Therefore, the wafer bowing can be measured by analyzing the discrepancy of growth rates at different positions on the wafer. Furthermore, the wafer bowings were confirmed by the ex-situ wafer bowing measurement. High-resistivity and low-resistivity Si substrates were used for epitaxial growth. In comparison with low-resistivity Si substrate, Ga N grown on high-resistivity substrate shows a larger wafer bowing caused by the highly compressive stress introduced by compositionally graded Al Ga N buffer layer. This transition of wafer bowing can be clearly in-situ measured by using the reflectivity mapping method.     

N2-plasma nitridation on Si(111): Its effect on crystalline silicon nitride growth

This study uses a combination of atom-resolved scanning tunneling microscopic and spectroscopic (STM/STS) techniques to investigate the effects of N₂-plasma nitridation on a crystalline β-Si₃N₄ ultrathin film grown on the Si(111) substrate. The proposed two-step growth process, including N₂-plasma nitridation followed by vacuum annealing both at high temperature (~ 900 °C), can substantially improve the atomic and electronic structures of the β-Si₃N₄ having an atomically uniform morphology and stoichiometry. The effects of nitridation and post-annealing temperatures were examined by monitoring the morphological and electronic–structural evolutions of a non-stoichiometric surface. Moreover, the two-step N₂-plasma nitridation process has extremely low activation energy and thus minimizes the thermal energy from the substrate for practical growth of β-Si₃N₄ ultrathin film.     

Preparation of SiO2 overlayers on oxide substrates by chemical vapor deposition of Si(OC2H5)4

The deposition of SiO₂ onto ZrO₂, TiO₂, MgO, SiO₂ and Al₂O₃ by chemical vapor deposition (CVD) of Si(OC₂H₅)₄ was studied by temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). TPD showed that Si(OC₂H₅)₄, adsorbed at 300 K, decomposed on ZrO₂, TiO₂, MgO and Al₂O₃ to give ethene and H₂O during the TPD. SiO₂ did not adsorb Si(OC₂H₅)₄ at 300 K. The decomposition of Si(OC₂H₅)₄ on ZrO₂ decreased as the amount of SiO₂ grew. The Zr AES signals attenuated strongly while that of Si increased. We conclude that a SiO₂ thin film about 10 Å thick forms on ZrO₂. This thin film was stable in vacuum up to 723 K but, at 823 K, either rearranged into small clusters or formed a solid solution with the surface of ZrO₂. The decomposition activity of TiO₂ for Si(OC₂H₅)₄ did not decrease strongly as SiO₂ deposition proceeded. This is accounted for if SiO₂ clusters are produced leaving TiO₂ sites exposed. Over both ZrO₂ and TiO₂ adsorption of CO₂ was suppressed by the deposition of SiO₂. There is no significant carbon buildup during deposition on ZrO₂ but on all the other oxides its accumulation is important.     

Pulsed laser deposition of Co and growth of CoSi2 on Si(111)

Monolayers of Co were prepared by pulsed laser deposition (PLD) and thermal deposition (TD) on Si(111) substrates. The surface structure and morphology were studied as a function of annealing temperature with scanning tunneling microscopy and low-energy electron diffraction (LEED). Comparing PLD to TD in the monolayer regime, we find surface phases with lower Co content in the top layer for annealing temperatures below 500 °C, indicating an implantation of Co into the Si substrate. The implanted Co leads to an increased intermixing of Co and Si and a higher density of nucleation centers for Co multilayers produced by PLD compared to TD. Multilayer PLD films therefore show an improved film quality, which is detected by an increased intensity of the LEED reflectivity. PACS 68.37.Ef; 61.14.Hg; 68.55.-a     

Transition from smectic-A to crystalline phase in the mean field approximation

We present here a smectic-A to crystalline phase transition curve for liquid crystals, in addition to the phase diagram previously reported which includes the isotropic, nematic, and smectic-A phases calculated in the mean field approximation.     

Thermal stability of Mg2Si epitaxial film formed on Si（111） substrate by solid phase reaction

A single crystalline Mg2 Si film was formed by solid phase reaction(SPR) of a Si(111) substrate with an Mg overlayer capped with an oxide layer(s),which was enhanced by post annealing from room temperature to 100 ℃in a molecular beam epitaxy(MBE) system.The thermal stability of the Mg2 Si film was then systematically investigated by post annealing in an oxygen-radical ambient at 300℃,450℃ and 650 ℃,respectively.The Mg2 Si film stayed stable until the annealing temperature reached 450 ℃ then it transformed into amorphous MgO x attributed to the decomposition of Mg2 Si and the oxidization of dissociated Mg.     

Partially dissociative adsorption of water vapor on the Si(111) (7×7) surface

High-resolution vibrational electron energy-loss spectra have been measured of the Si(111) (7×7) surface exposed to water vapor at 300 K. Direct experimental evidence is given that water adsorption on the Si(111) (7×7) surface is partially dissociative.     

Comparison between Si(100) and Si(111) in the reaction with oxygen at high temperatures

In this Letter, both the dynamics and kinetics of the reaction of oxygen molecules on Si(100)p2 × 1 and Si(111)7 × 7 and 1× 1 surfaces are compared. In all three cases, two distinct adsorption channels were observed. For oxygen molecules with translational energies less than 0.08 eV, the initial sticking is not sensitive to the energy or the angle of incidence, but displays a high sensitivity to the surface structure. At higher energies, a second channel becomes effective. The initial sticking coefficient increases rapidly and scales with the normal component of the translational energy, but the dependence on surface structure is greatly diminished. The kinetics of SiO formation are qualitatively similar on all surfaces with slightly higher rates on Si(111).     

SiO production from Si(100) and (111) surfaces by reaction with O2 beams

Desorption kinetics of SiO in the reaction of O₂ with Si(100) and (111) surfaces were investigated at surface temperatures between 1000 and 1300 K by using a pulsed molecular beam technique. The gaseous SiO product was detected by a mass spectrometer above 1000 K. At temperatures lower than 1050 K, the SiO signal appeared with an induction time after the O₂ beam irradiation onto the surface, which means that the desorption occurs via sequential steps. The rate constants for two steps were obtained by a computer simulation of the relaxation waveforms of the SiO signal. The values obtained are in the same range as those of D'Evelyn et al. However, they are one order of magnitude larger than those of Yu and Eldridge. The activation energies for the two steps are 2.8 and 2.4 eV for both Si(100) and (111) surfaces. No significant difference between the two kinds of surfaces was found from these values. However, many etch pits were observed on the Si(100) surface after the reaction, while no such etch pits were formed on the Si(111) surface. The planes of the etch pits formed on the (100) surfaces consisted mostly of the {111} facets.