Dopant behavior in silicon-germanium (SiGe) alloys presents unique challenges and opportunities compared to pure silicon due to the interplay of strain, composition, and atomic-scale interactions. Understanding dopant activation, diffusion mechanisms, and segregation effects is critical for optimizing SiGe-based devices, particularly in high-speed electronics, heterojunction bipolar transistors, and advanced CMOS technologies.

**Dopant Activation in SiGe Alloys**
Dopant activation refers to the process by which impurity atoms incorporate into the lattice and become electrically active donors or acceptors. In SiGe, activation efficiency depends on Ge concentration, strain, and thermal processing. Boron, a common p-type dopant, exhibits higher activation in SiGe than in silicon due to reduced formation energy of substitutional boron in the presence of compressive strain. For example, in Si₀.₈Ge₀.₂, boron activation can be 10-20% higher than in silicon at equivalent doping concentrations. Phosphorus and arsenic, n-type dopants, show more complex behavior. While phosphorus activation remains relatively stable up to moderate Ge fractions (≤30%), arsenic activation decreases due to increased defect-assisted deactivation.

The presence of Ge alters the density of states and Fermi level positioning, influencing dopant solubility. Heavy doping (≥1×10²⁰ cm⁻³) in SiGe often leads to incomplete activation due to cluster formation, particularly for n-type dopants. Post-implantation annealing plays a crucial role, with optimal temperatures typically 50-100°C lower than in silicon to avoid excessive diffusion while achieving high activation.

**Diffusion Mechanisms in SiGe**
Diffusion in SiGe alloys is governed by interactions between dopants, point defects, and the local strain field. Boron diffusion is notably suppressed in compressively strained SiGe due to reduced interstitial-mediated diffusion. In relaxed SiGe, boron diffusivity increases with Ge content, following a non-linear relationship. For instance, in Si₀.₇Ge₀.₃, boron diffusivity can be 2-3 times higher than in silicon at 900°C.

Phosphorus and arsenic diffusion exhibit opposite trends. Phosphorus diffusivity decreases in SiGe due to stronger binding with vacancies, while arsenic shows enhanced diffusion at high Ge concentrations (>40%) due to strain-induced changes in the vacancy formation energy. Antimony, a slower diffuser in silicon, becomes even less mobile in SiGe, making it attractive for precise doping profiles.

Interdiffusion of Si and Ge atoms themselves must also be considered during thermal processing. At temperatures above 800°C, Ge tends to segregate, leading to composition modulation unless carefully controlled. This interdiffusion is mediated by vacancies and has an activation energy of ~4.5 eV in relaxed alloys, increasing under strain.

**Segregation Effects**
Segregation occurs when dopants preferentially distribute between Si and Ge regions during growth or annealing. Boron exhibits segregation toward Si-rich regions due to its lower solubility in Ge. The segregation coefficient (ratio of concentration in Si to Ge) ranges from 2-5 depending on temperature and strain. This effect is exploited in heterostructure devices to create sharp doping profiles.

N-type dopants like phosphorus and arsenic segregate toward Ge-rich regions, though less pronounced than boron. Arsenic segregation becomes significant above 20% Ge content, with coefficients of 0.3-0.6. This behavior stems from differences in bond strength and strain relaxation around the dopant atom.

At interfaces, segregation can lead to dopant accumulation or depletion. In Si/SiGe heterojunctions, boron piles up on the Si side while phosphorus accumulates on the Ge side. This interfacial doping asymmetry must be accounted for in device design, particularly in modulation-doped structures where carrier confinement relies on precise dopant positioning.

**Contrast with Silicon Doping Dynamics**
The key differences between SiGe and silicon doping can be summarized as follows:

1. **Strain Dependence**: SiGe doping is strongly influenced by strain, whereas silicon doping is relatively insensitive to lattice deformation. Compressive strain in SiGe enhances boron activation but suppresses diffusion, effects absent in silicon.
2. **Compositional Effects**: Ge content directly modifies defect thermodynamics in SiGe, unlike homogeneous silicon. Vacancy formation energies decrease with increasing Ge fraction, altering diffusion pathways.
3. **Segregation**: Dopant redistribution between Si and Ge regions has no equivalent in pure silicon, requiring new models for junction formation.
4. **Temperature Sensitivity**: Optimal annealing windows for SiGe are narrower due to competing effects of activation and interdiffusion, unlike the more forgiving silicon processing.

**Practical Implications for Device Engineering**
The distinct doping characteristics of SiGe enable novel device architectures but demand precise control. Strain-engineered SiGe channels in pMOSFETs benefit from enhanced hole mobility and suppressed boron diffusion, enabling shallower junctions. In HBTs, careful management of phosphorus segregation ensures optimal emitter efficiency. For power devices, the thermal stability of antimony profiles in SiGe allows robust high-temperature operation.

Future developments may exploit these doping asymmetries for atomically precise devices, leveraging segregation effects at monolayer interfaces. The continued scaling of SiGe technologies will require advanced doping techniques such as delta doping and laser-assisted activation to overcome limitations imposed by conventional thermal processing.

In summary, doping in SiGe alloys represents a complex interplay of chemistry, strain, and defect physics that differs fundamentally from silicon. Mastery of these effects has been instrumental in advancing high-performance semiconductor devices and will remain critical for next-generation heterostructure technologies.