Maximum-entropy mobility spectrum of two-dimensional hole gas in strained-Si₁-ₓGeₓ/Si heterostructures
Magnetotransport properties of modulation-doped p-type Si₁-ₓGeₓ/Si and Si₁-ₓGeₓ/Si₁₋yGey heterostructures were studied, in the magnetic field range 0-12 T, and in the temperature range 0.35-300 K. The experimental data within the classical regime have been analysed by mobility spectrum analysis, in order to separate the influences of different parallel conduction paths. A new method of mobility spectrum analysis has been developed by the author, based on the concept of maximum-entropy, and this computation has been shown to overcome several drawbacks or limitations of previous mobility spectrum methods of calculation. The data have also been analysed by Beck & Anderson's analysis and the multicarrier fitting method for comparison. Analysis of the magnetic-field-dependent resistivity tensors reveals a two-dimensional hole gas (2DHG) in the Si/SiGe/Si quantum well, carriers in the boron-doped cap layer, and an unknown electron-like carrier. The carrier density of the 2DHG can either remain constant (x = 0.1), increase (x = 0.13), or decrease (x ≥ 0.2), with increasing temperatures. Differences in the temperature dependences are partly attributed to different growth conditions. A decreasing carrier density with increase in temperatures may indicate the presence of acceptor-like defect states near the valence band edge of the SiGe channel. The mobility of the 2DHG between 100-300 K has the form AT⁻γ and γ has the bowl shape with the minimum at x ~ 0.25-0.3. These characteristics suggest a possible influence of alloy disorder scattering. The mobilities and activation energies of the carriers in the boron-doped cap vary between samples and this is believed to be due to boron-spike near the Si/Si-substrate interface, in some samples. The source of electron-like carrier is presently unknown.