| dc.description.abstract | In conclusion, we have shown for the first time that the parallel conduction and other kinds of parasitic contributions to the channel mobility can be reduced effectively by hydrogen passivation, and an enhancement in the room-temperature mobilities up to ~2000 cm²/V·s can be achieved in the case of samples with large defect densities and background impurities. Unlike the chemical etching technique, this method is non-destructive and reversible. We have also shown that both plasma as well as electrolytic methods of hydrogen passivation can be used to remove parallel conduction, though the plasma process is more effective. We have established that the control sample can be recovered from the hydrogenated sample by annealing in vacuum at elevated temperatures.
In conclusion, we have shown an enhancement in the room-temperature mobility of 2DHG in the Si???Ge? channels by reducing the parallel conduction and other kinds of parasitic contributions to the channel mobility by hydrogen passivation. The low-temperature conductivities of the hydrogenated sample show an activated behavior below 50 K, and the mobility degrades by an order of magnitude, indicating a possible localization of the 2D carriers. Such localization behavior is possibly caused by hydrogen-induced disorder by the plasma hydrogenation process and is confirmed by annealing the sample in vacuum.
Though extracted hole mobilities in the range of 500–600 cm²/V·s are shown by us in the case of hydrogen-passivated, low-Ge-content SiGe channels and 800–1000 cm²/V·s in the case of high-Ge-content SiGe channels by Ismail et al. (Ismail et al., 1994), they still lag behind the electron mobilities by a factor of two at room temperature and an order of magnitude at low temperature. This is non-ideal if one has to fabricate SiGe-based complementary metal-oxide-semiconductor (CMOS) devices. In present-day Si-CMOS technology, in order to match the current drives of n-channel MOS and p-channel MOS, the area of the PMOS device is taken about 2–3 times larger than NMOS. This adversely affects the level of integration and device speed. Hence, it is very important to further improve the PMOS device speeds in order to make them comparable to NMOS performance. The SiGe HBT exhibited very high performance in high-frequency operation, but the improvement of current drivability in a 0.25 ?m channel MOSFET was relatively small (20%) though channel mobility was improved by 50%. The reason for this is the onset of velocity saturation as a consequence of low hole mobility (<300 cm²/V·s). Hole mobility enhancement of greater than ~1000 cm²/V·s is therefore necessary for making very high-speed CMOS. In the next chapter, we discuss the use of pure Ge channels in modulation-doped heterostructures to achieve this goal.
In summary, we have demonstrated record room-temperature hole mobility of ~3000 cm²/V·s in Ge-channel 2DHG by reducing the parallel conduction and remote impurity scattering after hydrogen passivation. We have shown that the low-temperature mobility decreases after hydrogenation, indicating a possible plasma-induced disorder. In contrast to the low hole mobility SiGe-channel 2DHG, no insulating behavior was observed in the case of plasma-exposed Ge-channel 2DHG. This is attributed to the high mobility and very low effective mass of holes. Finally, in an attempt to study the mobility dependence on carrier density, we have done photo-Hall measurements and confirmed that impurity scattering dominates the low-temperature hole mobility in the 2D channel.
The last few years have seen a tremendous improvement in Si/Si???Ge? heterostructure material research, resulting in improved high-quality channels for high-performance devices like field-effect transistors (FETs), bipolar junction transistors (BJTs), and modulation-doped field-effect transistors (MODFETs). There have been increased efforts in improving the material quality to enhance the electron and hole mobilities to reach theoretical limits. Though record two-dimensional electron gas (2DEG) mobility of ~2800 cm²/V·s was reported at room temperature, typical room-temperature electron mobilities range from 1600 to 2000 cm²/V·s, limited mainly by parallel conduction. Similarly, observed room-temperature two-dimensional hole gas (2DHG) mobilities are still much below the theoretical values. In an attempt to improve the carrier mobilities, in Chapters 2 and 3, we used plasma and electrolytic hydrogen-passivation techniques and successfully reduced the parallel channel’s carrier density, thereby increasing the room-temperature mobilities of the 2DEG in strained Si and 2DHG in strained SiGe channels. While excessive carriers in the supply layer (contributing to Coulombic scattering), unintentional dopants in the channel (contributing to ionized impurity scattering) are neutralized and other defects and dislocations are passivated, hydrogenation has led to interesting results in terms of low-temperature mobilities of 2DHG in p? Si/strained SiGe heterostructures. In Chapter 4, similar passivation studies were carried out on 2DHG in strained Ge-channel. The results showed a great improvement in the room-temperature mobilities to as high as 3000 cm²/V·s from a control sample value of ~1200 cm²/V·s, but the corresponding carrier density turned out to be higher than the control sample. This we attributed to the parasitic contribution from the highly doped n-Si substrate. Since there is more than one parasitic channel, the two-carrier model could not be used to extract the channel values. In order to study the mobility dependence on carrier density, photo-Hall measurements were carried out at 10 K by illuminating the sample with an LED. By controlling the power output onto the sample, the carrier density in the channel is changed. It was found that the mobility increases with channel density as ? ? n? with a ? 2. The sample was found to be very sensitive to incident light power. A detailed study of the photoconductivity measurements on this sample was carried out in Chapter 5. Photoresponse in modulation-doped Si/Ge?Si???/Ge/Ge?Si??? two-dimensional hole gas (2DHG) heterostructures at low temperatures is presented in Chapter 5. It was found that the sample’s response to light (from ? = 0.5 to 2.0 ?m) was extremely sensitive. In a simple two-probe resistance measurement at low temperature (~10 K), it was shown for the first time that the resistance of the 2DHG decreases significantly upon illumination with incident powers as low as ~1 femtowatt. The sensitivity of the sample, defined as the ratio of the change in sample’s resistance to exposure time, was measured over several orders of incident powers from ~1 fW to 10 nW and was found to change sub-linearly with power. The measured quantum efficiency exceeded the theoretical limit by a factor of ~10. A simple model involving ground and excited states of the impurities and the dominant Auger recombination processes between them is presented to explain the observations.
When the illumination onto the sample is turned off, the reduced sample resistance recovers partially to the dark value, indicating persistency in photoconductivity (PPC). This was explained by the photoexcitation of electron-hole pairs in the Ge?Si??? layers and their subsequent spatial separation by the electric field in the heterojunction. The holes eventually get collected into the 2D channel, as evidenced by the increase in the Hall density under illumination. The recombination of
Electrons and holes are inhibited from recombining by the heterojunction barrier, causing the persistent photoconductivity (PPC). When the illumination is turned off, the decay of the enhanced conductivity in our samples was found to be a non-exponential function of time and can be fitted to a logarithmic function. This indicates that the holes in the 2DHG eventually recombine by tunneling through spatially distributed traps. The PPC effect disappears at temperatures T > 200 K.
This effect, combined with the ultra-sensitive photoresponse, has potential applications in image-sensing devices like Si charge-coupled device (CCD) arrays, although with much higher quantum efficiency. | |
