J. O. Chu

Measurements of alloy composition and strain in thin GexSi1−x layers

The utility of Raman spectroscopy for the simultaneous determination of composition and strain in thin GexSi1−x layers has been investigated. Using data from the literature and new data for the strain shift of the Si‐Si phonon mode presented here, we show how Raman spectra provide several different means of measuring composition and strain in samples as thin as 200 A. We demonstrate that for largely relaxed layers with compositions near x=0.30, Raman scattering can measure the composition, x, with an accuracy of ±0.015 and the strain, e, with an accuracy ±0.0025. The accuracy of the alloy composition obtained from Raman spectra is comparable or, in the case of very thin layers, superior to that measured by other techniques such as x‐ray diffraction, electron microprobe, and Auger electron spectroscopy.

Extremely high electron mobility in Si/SiGe modulation‐doped heterostructures

We report record high electron mobility in modulation‐doped Si/SiGe. Samples grown by ultrahigh vacuum chemical vapor deposition (UHV‐CVD) with mobility values in the range of 3.2–5.2×105 cm2/Vu2009s have been measured at 0.4 K. The current and temperature dependence of the magnetoresistance in those samples have been examined and the scattering times are deduced from these measurements. At high magnetic field (≳10 T), fractional quantum Hall filling factors have been observed, and the corresponding activation energies have been calculated. These are significantly larger than previously reported values in Si/SiGe, and are comparable to those in GaAs/AlGaAs modulation‐doped heterostructures with mobility higher than 1×106 cm2/Vu2009s.

Electron transport properties of Si/SiGe heterostructures: Measurements and device implications

We report electron transport properties of modulation‐doped Si/SiGe at 300 and 77 K. Record mobilities of 2830 and 18u2009000 cm2/Vu2009s at 300 and 77 K, respectively, have been measured. Depending on the spacer layer thickness, the sheet resistance of the Si channel is in the range of 2000–10u2009000 Ω/⧠ at 300 K and 450–700 Ω/⧠ at 77 K. The low field electron drift velocity is 2–3 (5–10) times higher than the corresponding velocity measured in Si/SiO2 structures at 300 K (77 K). The saturation velocity is measured to be only 5% higher than in bulk Si, at both 300 and 77 K, but appears at a lower electric field. The effect of the enhanced transport properties in modulation‐doped Si/SiGe on device design and performance is investigated.

SiGe-on-insulator prepared by wafer bonding and layer transfer for high-performance field-effect transistors

SiGe-on-insulator material was fabricated by wafer bonding and hydrogen-induced layer transfer techniques. The transferred SiGe layer is strain relaxed and has a Ge content ranging from 15% to 25%. High-quality strained Si layers were grown on the SiGe-on-insulator substrates by the UHV/chemical vapor deposition process at 550u200a°C. An electron mobility of 40u200a000u200acm2/Vu200as in a modulation-doped Si/SiGe heterostructure was achieved at 30 K on a SiGe-on-insulator substrate.

High hole mobility in SiGe alloys for device applications

We report high hole mobility in modulation‐doped SiGe alloys with Ge content up to 80%. The layers which are grown using ultrahigh‐vacuum chemical vapor deposition are of high crystalline quality, have smooth surfaces, and have a low density of misfit dislocations. As a result of strain and high Ge content, we have measured hole mobilities in the range of 800–1050 cm2/Vu2009s at room temperature, and 3300–3500 cm2/Vu2009s at 77 K. The corresponding two‐dimensional sheet hole density is about 3×1012 cm−2. Those numbers are, to our knowledge, the highest numbers ever reported for a SiGe alloy. The resistivity of this two‐dimensional hole channel at room temperature is, to our knowledge, the lowest for any p‐type semiconductor quantum well.

High-transconductance n-type Si/SiGe modulation-doped field-effect transistors

The authors report on the fabrication and the resultant device characteristics of the first 0.25- mu m gate-length field-effect transistor based on n-type modulation-doped Si/SiGe. Prepared using ultrahigh vacuum/chemical vapor deposition (UHV/CVD), the mobility and electron sheet charge density in the strained Si channel are 1500 (9500) cm/sup 2//V-s and 2.5*10/sup 12/ (1.5*10/sup 12/) cm/sup -2/ at 300 K (77 K). At 77 K, the devices have a current and transconductance of 325 mA/mm and 600 mS/mm, respectively. These values far exceed those found in Si MESFETs and are comparable to the best results achieved in GaAs/AlGaAs modulation-doped transistors.<<ETX>>

High-speed Germanium-on-SOI lateral PIN photodiodes

We report the fabrication and characterization of high-speed germanium on silicon-on-insulator lateral PIN photodetectors. At an incident wavelength of 850 nm, 10 /spl times/10-/spl mu/m detectors with finger spacing S of 0.4 /spl mu/m (0.6 /spl mu/m) produced a -3-dB bandwidth of 29 GHz (27 GHz) at a bias voltage of -1 V. The detectors with S=0.6 /spl mu/m had external quantum efficiency of 34% at 850 nm and 46% at 900 nm and dark current of 0.02 /spl mu/A at -1-V bias.

Room‐temperature electron mobility in strained Si/SiGe heterostructures

We report on room‐temperature electron transport measurements in modulation‐doped strained Si/SiGe heterostructures, grown by ultrahigh‐vacuum chemical vapor deposition. A high room‐temperature mobility is expected in such samples because of the strain‐induced splitting of the conduction band in the silicon channel. Record values of over 2600 cm2/Vu2009s have been measured, almost twice the theoretical maximum for relaxed silicon.

Raman scattering analysis of relaxed GexSi1−x alloy layers

We have used Raman scattering to evaluate thick epitaxial GexSi1−x layers with 0.20≤x≤0.43 grown on Si (100) substrates. We show that a detailed consideration of the composition dependencies of the relative intensities of the various phonon modes can enhance the sensitivity of Raman scattering to variations in composition and strain. We find that samples are uniform on a scale of ≂1 μm laterally and <1000 A in the growth direction.

Extremely high transconductance Ge/Si/sub 0.4/Ge/sub 0.6/ p-MODFET's grown by UHV-CVD

Ge-channel modulation-doped field-effect transistors (MODFETs) with extremely high transconductance are reported. The devices were fabricated on a compressive-strained Ge/Si/sub 0.4/Ge/sub 0.6/ heterostructure with a Hall mobility of 1750 cm/sup 2//Vs (30,900 cm/sup 2//Vs) at room temperature (77 K). Self-aligned, T-gate p-MODFETs with L/sub g/=0.1 /spl mu/m displayed an average peak extrinsic transconductance (g(m/sub ext/)) of 439 mS/mm, at a drain-to-source bias voltage (V/sub ds/) of -0.6 V, with the best device having a value of g(m/sub ext/)=488 mS/mm. At 77 K, values as high as g(m/sub ext/)=687 mS/mm were obtained at a bias voltage of only V/sub ds/=-0.2 V. These devices also displayed a unity current gain cutoff frequency (f/sub T/) of 42 GHz and maximum frequency of oscillation (f/sub max/) of 86 GHz at V/sub ds/=-0.6 V and -1.0 V, respectively.