Search

We report the development of a new method to systematically and controllably achieve very high carrier concentrations in As-doped germanium using ultra-low temperature, high efficiency routes based on the structurally and chemically compatible inorganic hydrides As(SiH3)3 and As(GeH3)3. The Ge n-layers are grown on Ge-buffered Si(100) using in situ depositions of the compounds with Ge3H8 at 330 °C. The as-grown films are found to exhibit excellent crystallinity, defect-free interfaces, atomically smooth surfaces and flat doping profiles with abrupt edges. The active carrier densities are measured to be in the range of 1 × 1019–8.4 × 1019 cm−3 irrespective of the precursor type. These carrier densities are in close agreement with atomic As concentrations measured by secondary ion mass spectrometry, indicating that the growth mechanism promotes the nearly complete substitutional incorporation of dopant atoms while suppressing the formation of non-active clusters and defects. In spite of the lower solubility of As in Ge relative to that of P, the maximum carrier concentrations obtained with As(SiH3)3 and As(GeH3)3 are roughly 30% higher than those found with the analogous P(SiH3)3 and P(GeH3)3. This result, along with the close similarity in band gap narrowing observed for the two methods, suggests that the As-doping route may be advantageous for optical devices that require the highest possible carrier concentrations to populate the conduction band valley associated with direct gap emission. On the other hand—due to the inherently shorter carrier relaxation times in As-doped Ge—the lowest observed resistivity of 5 × 10−4 Ω cm is slightly higher than the lowest resistivity from P-doped analogs. Finally, optical responsivity, electroluminescence and I–V properties of photodiodes fabricated using As(SiH3)3 and As(GeH3)3 are found to be on par with those observed from Ge-on-Si reference analogs, indicating that the chemistry approach described here represents a viable and straightforward route to doping and activation of device-quality materials. [ABSTRACT FROM AUTHOR]

Ge–Si based n-type films are synthesized using specially designed hydrides P(SiH3)3, Ge3H8 and Ge4H10 for potential applications in next-generation CMOS technologies. The films are grown on Ge buffered Si(100) at 340 °C using two complementary methods. The first employs a gas-source molecular epitaxy approach using Ge4H10 to produce materials with P doping densities varying from 4 × 1018 to a 3.5 × 1019 cm−3 threshold. These materials are co-doped with Si concentrations ranging from 3 × 1019 cm−3 to 3.5%, roughly in proportion with the amount of P(SiH3)3 used in the reactions. The second approach applies an alternative ultra-high vacuum chemical vapor deposition (UHV–CVD) technique and Ge3H8 in place of Ge4H10 to achieve ultra-high carrier concentrations up to ∼6 × 1019 cm−3. The Si content in this case is minimal—in the 2–6 × 1019 cm−3 range—indicating that the growth mechanism allows only ‘impurity’ levels of Si to be incorporated. The active carrier densities in both cases closely reflect the absolute P content, indicating that the P atoms are mostly substitutional. The electron mobilities are significantly higher compared to state-of-the-art prototypes, probably due to superior microstructure and dearth of inactive donors in the lattice. P–I–N diodes fabricated using the P(SiH3)3 compound show I–V characteristics comparable to state-of-the-art results for Ge-on-Si devices and are virtually undistinguishable from similar diodes doped with the P(GeH3)3 precursor. These results confirm P(SiH3)3 as a viable CVD doping source that is practical from a process standpoint and therefore attractive for industrial scale-up. [ABSTRACT FROM AUTHOR]

The compositional dependence of the lowest direct and indirect band gaps in Ge1−ySny alloys has been determined from room-temperature photoluminescence measurements. This technique is particularly attractive for a comparison of the two transitions because distinct features in the spectra can be associated with the direct and indirect gaps. However, detailed modeling of these room temperature spectra is required to extract the band gap values with the high accuracy required to determine the Sn concentration yc at which the alloy becomes a direct gap semiconductor. For the direct gap, this is accomplished using a microscopic model that allows the determination of direct gap energies with meV accuracy. For the indirect gap, it is shown that current theoretical models are inadequate to describe the emission properties of systems with close indirect and direct transitions. Accordingly, an ad hoc procedure is used to extract the indirect gap energies from the data. For y < 0.1 the resulting direct gap compositional dependence is given by ΔE0 = −(3.57 ± 0.06)y (in eV). For the indirect gap, the corresponding expression is ΔEind = −(1.64 ± 0.10)y (in eV). If a quadratic function of composition is used to express the two transition energies over the entire compositional range 0 ≦̸ y ≦̸ 1, the quadratic (bowing) coefficients are found to be b0 = 2.46 ± 0.06 eV (for E0) and bind = 1.03 ± 0.11 eV (for Eind). These results imply a crossover concentration yc = , much lower than early theoretical predictions based on the virtual crystal approximation, but in better agreement with predictions based on large atomic supercells. [ABSTRACT FROM AUTHOR]