Silicon-germanium Alloy Thin Films Research Articles

Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films

Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si1-xGex:H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors.

Impact of strain on hole mobility in the inversion layer of PMOS device with SiGe alloy thin film

Advanced MOSFET devices formed from Si-based materials, such as silicon-germanium alloys, are simple and low cost to manufacture. This work focuses on hole mobility in the inversion layer of PMOSFETs using alloy channel materials. The primary topic of this work is the theoretical calculation of effective mass and hole mobility in the silicon-germanium alloy p-type metal-oxide-semiconductor field-effect transistor (PMOSFET) inversion layer. The strain conditions considered in the calculations are intrinsic strain resulting from growing the silicon-germanium alloy thin films on the three orientation Si substrates. The hole mobility of silicon-germanium alloy inversion layer for PMOSFET under substrate strain and various germanium mole fractions are all investigated. The impact of wafer orientation and channel direction on the hole mobility is analyzed using the Kubo–Greenwood formalism.

Experimental Investigation of Size Effects on the Thermal Conductivity of Silicon-Germanium Alloy Thin Films

We experimentally investigate the role of size effects and boundary scattering on the thermal conductivity of silicon-germanium alloys. The thermal conductivities of a series of epitaxially grown Si(1-x)Ge(x) thin films with varying thicknesses and compositions were measured with time-domain thermoreflectance. The resulting conductivities are found to be 3 to 5 times less than bulk values and vary strongly with film thickness. By examining these measured thermal conductivities in the context of a previously established model, it is shown that long wavelength phonons, known to be the dominant heat carriers in alloy films, are strongly scattered by the film boundaries, thereby inducing the observed reductions in heat transport. These results are then generalized to silicon-germanium systems of various thicknesses and compositions; we find that the thermal conductivities of Si(1-x)Ge(x) superlattices are ultimately limited by finite size effects and sample size rather than periodicity or alloying. This demonstrates the strong influence of sample size in alloyed nanosystems. Therefore, if a comparison is to be made between the thermal conductivities of superlattices and alloys, the total sample thicknesses of each must be considered.

Properties of Hydrogenated Amorphous Silicon-Germanium Alloys Deposited by Dual Target Reactive Magnetron Sputtering

ABSTRACTHydrogenated amorphous silicon-germanium alloy thin films (a-Si1-xGex:H) were deposited using reactive magnetron sputtering. Dual targets of silicon and germanium were sputtered in an argon + hydrogen atmosphere using rf excitation. Films with x = 0.4 were deposited as a function of substrate temperature and hydrogen partial pressure, and were evaluated by dark and photoconductivity, infrared absorption, and optical transmission. Photosensitivity reached a maximum value of about 5000 between 150 and 200 °C. Using the stretching modes in the region of 2000 cm-1, the hydrogen bonding was characterized in terms of the preferential attachment ratio (PA), which represents the ratio of H bonded to silicon to that bonded to germanium. The PA shows a systematic increase with increasing temperature, independent of hydrogen partial pressure. The interplay between thermodynamic and kinetics effects in determining PA and film quality will be discussed.

Correlation of structural inhomogeneities with transport properties in amorphous silicon germanium alloy thin films

Amorphous silicon–germanium (a-Si 1− x Ge x :H) alloy thin films were studied over a wide range of Ge content ( x=0–1.00) grown by radio frequency plasma CVD (rf PECVD) technique with variation of different deposition parameters such as H 2 dilution, rf power and square wave pulse modulation (SWPM) of rf amplitude. Structural properties like microstructure factor ( R ⁎) and AFM surface roughness ( R RMS ) were correlated with the transport properties such as mobility-lifetime product ( μτ) and ambipolar diffusion length ( L d ) of these films. Near the middle composition range ( x=0.32–0.70), the R ⁎ in these films varies between 0.20 and 0.42 and L d ranges between 50 and 60 nm. Films deposited near the pure silicon and pure germanium ends have improved structural and transport properties. By SWPM method we have been able to significantly lower the R ⁎ value of the a-Si 1− x Ge x :H films to 0.15 with x=0.40–0.45 resulting in L d =100 nm and μτ=1.0×10 −6 cm 2 V −1.

The effects of native and light induced defects on optoelectronic properties of hydrogenated amorphous silicon–germanium (a-SiGe:H) alloy thin films

Effects of native and light induced defects states in hydrogenated amorphous silicon–germanium alloy thin films with different Ge concentrations have been investigated by using steady-state photoconductivity, dual beam photoconductivity (DBP), transmission spectroscopy and photothermal deflection spectroscopy (PDS) techniques. In the annealed state, sub-bandgap absorption spectra obtained from both PDS and DBP overlap very well at energies above 1.4 eV. However, differences in α (hν) spectrum exist in the lower energy part of absorption spectrum. The α (hν) value measured at 1.0 eV is the lowest for 10% Ge sample and increases gradually as Ge content of the sample increases. In the light soaked state, time dependence of photoconductivity decay obeys to t −x power law, where x changes from 0.30 to 0.60 for samples with low Ge content and 0.05–0.1 for samples with high Ge content. Correspondingly, the increase of the sub-bandgap absorption coefficient at lower energies obeys to t y power law, where y values are lower than the x value of the same sample. It can be inferred that sub-bandgap absorption and photoconductivity measurements are not controlled by the same set of defects created in the bandgap of alloys.

Structural and optoelectronic properties of silicon germanium alloy thin films deposited by pulsed radio frequency plasma enhanced chemical vapor deposition

Hydrogenated amorphous silicon germanium (a-SiGe:H) alloy films are still under study to improve their incorporation into tandem solar cells. In this paper we have investigated the quality of films deposited in a radio frequency powered plasma enhanced chemical vapor deposition unit. Two series of samples were prepared either from a mixture of silane and germane diluted into hydrogen or from the same mixture to which a small amount of argon was added. The applied rf power at 13.56 MHz was either continuous or modulated by a square wave pulse at a frequency of 1356 kHz. Different films were deposited in the two series controlling the time the plasma was “on.” The ratio of the on time to the total period of the modulation, the duty cycle, was varied between 100% (continuous mode) and 50%. The plasma during deposition as well as structural and optoelectronic properties of the resulting films was studied with several techniques. We found an optimum in the transport properties for a duty cycle of 75%: an ambipolar diffusion length of the order of 100 nm for a material presenting a bandgap of 1.44 eV. To explain this promising result, we present a discussion on the growth mechanisms of such layers linking the structural results to the transport data.

Microstructural and Radial Distribution Function Analysis of Hydrogenated Silicon, Germanium, and Silicon-Germanium Alloy Thin Films

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

High diffusion length silicon germanium alloy thin films deposited by pulsed rf PECVD method

Hydrogenated amorphous silicon germanium (a-Si1−xGex:H) alloy thin films were deposited in a rf (13.56MHz) powered plasma CVD system from a mixture of silane, germane and hydrogen. The rf power was pulsed at a frequency of 1356Hz with the duty cycle (Q) ranging from 50% to 100%. The Ge content in the films, found in the range 40–45%, was deduced from Raman spectra. The band gaps Eg of the films were in between 1.44 and 1.48eV. We find a good correlation between the ambipolar diffusion length (Ld) and the microstructure factor (R∗) for different duty cycles. At Q=75% the R∗ value is lowest (0.14) which correlates well with the maximum value of Ld (100nm), the highest Ld value reported for a-Si1−xGex:H films with Eg = 1.44eV. It was also found to be very stable under light soaking. Powder incorporation within the films was also studied. Quality of the films is determined by two opposing effects viz. size of the powders incorporated into the films from the plasma and the bombardment of energetic ions and neutrals. Best transport properties were found at an optimum duty cycle of 75%.

Conductance fluctuations in undoped hydrogenated amorphous silicon–germanium alloy thin films

We report coplanar conductance fluctuations of device quality, undoped hydrogenated amorphous silicon–germanium alloy thin films (a-SiGe:H) measured from 430 to 490 K. The a-SiGe:H alloys produce noise power spectra similar to coplanar undoped a-Si:H films in the same temperature range. The noise power spectrum S n does not fit a single 1/ f α power law but rather has two distinct regions, each accurately fitted by a power law, but with different slopes. The low frequency slope α 1 is similar to that observed in undoped a-Si:H films varying from 1.30 to 1.46 for different Ge concentrations and shows a slight temperature dependence. At higher frequencies, the slope α 2 is less than unity and temperature independent but depends on the Ge content of the film. α 2 decreases from 0.60 for no Ge (pure a-Si:H) to 0.15 for 40 at.% Ge. The noise power at lower frequencies increases and at higher frequencies decreases substantially as the temperature increases from 430 to 490 K. We infer that similar noise mechanisms are operating in undoped a-SiGe:H and a-Si:H films but that the Ge content is influencing the noise, particularly the slope at higher frequencies. In addition, the noise has the expected quadratic dependence on bias current and obeys Gaussian statistics.