K. Ismail

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/V s 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/V s.

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 18 000 cm2/V s 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 2000–10 000 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.

High electron mobility in modulation-doped Si/SiGe

Ultrahigh‐vacuum chemical vapor deposition has been exploited to grow single‐heterojunction n‐type modulation‐doped Si/SiGe structures. Phosphorus dopant is imbedded in the SiGe layer at two distinct positions: one at the surface to prevent depletion by surface states, and the other separated from the Si heterointerface by an intrinsic SiGe spacer, to supply electrons to the two‐dimensional electron gas. With a 4‐nm‐thick spacer layer, peak mobilities of 1800 cm2/V s, 9000 cm2/V s, and 19 000 cm2/V s were measured at room temperature, 77 and 1.4 K, respectively. These are the highest values reported for this material system.

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/V s at room temperature, and 3300–3500 cm2/V s 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.

Electron resonant tunneling in Si/SiGe double barrier diodes

We report upon the fabrication and characterization of the first n‐type resonant tunneling diodes in the SiGe materials system. The devices fabricated were Si/SiGe/Si/SiGe/Si double‐barrier diodes, employing strain‐relieved SiGe as the barrier layers surrounding pseudomorphic tensile strained Si. These devices were prepared using ultrahigh vacuum chemical vapor deposition. Negative differential conductance is observed at room temperature in these devices with a peak‐to‐valley ratio of 1.2. The corresponding value at 77 K is 1.5.

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>>

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/V s have been measured, almost twice the theoretical maximum for relaxed silicon.

High-performance Si/SiGe n-type modulation-doped transistors

Enhancement-mode Si/SiGe n-type modulation-doped transistors with a 0.5- mu m-length T-gate have been fabricated. Peak transconductances of 390 mS/mm at room temperature and 520 mS/mm at 77 K have been achieved. These high values are attributable to a combination of the high quality of the material used, having a room temperature mobility of 2600 cm/sup 2//V-s at an electron sheet concentration of 1.5*10/sup 12/ cm/sup 2/, and an optimized layer design that minimizes the parasitic series resistance and the gate-to-channel distance.<<ETX>>

Relaxed Si0.7Ge0.3 buffer layers for high‐mobility devices

The minimum epitaxial layer thickness required to produce relaxed, thermally stable, Si0.7Ge0.3 buffer layer structures for high electron‐ and hole‐mobility devices has been determined, using high resolution x‐ray diffraction. A 1.4‐μm‐thick layer, step graded to x=0.35, is sufficiently thick so that the residual strain in a uniform composition Si0.33Ge0.67 layer grown on top of it is essentially independent of thickness or growth temperature of the layer. Such structures are stable when annealed at 750 °C.

Modulation‐doped n‐type Si/SiGe with inverted interface

We report the growth of n‐type modulation‐doped Si/SiGe with the doped SiGe supply layer underneath the strained Si channel. The mobility and charge density are measured in samples with 2‐ and 3‐nm‐thick spacers using gated Hall measurements. A peak room temperature mobility of 2200 cm2/V s is measured at a sheet density of 2.5×1012 cm−2. The measurements indicate a clear mobility modulation especially near threshold. Our layer design allows the gate to induce a sheet charge density of up to 3.2×1012 cm−2, before any significant reduction in the mobility is observed.