Electron Transport Properites Of Novel Semiconductor Materials

In this paper, we report the investigation on the novel semiconductor materials of silicon quantum dots and Indium nitride. We mainly demonstrate the study on the electron transport properities of three kinds of different structures: Silicon quantum dots (nc-Si:H)/crystalline Silicon (c-Si) heterojunction diode, Indium nitride (InN) thin film and Indium nitride/Gallium arsenide (GaAs) interface according to the theoretical analysis with the aid of direct current (dc) and alternating current (ac) measurement.Generally, there exist electron channel and two dimensional (2D) electron gas (EG) at the heterojunction interface due to the conduction-band discontinuity. Similarly, there is 2DEG existed at nc-Si:H/ c-Si interface, which has a sheet density of about 1011 cm-2. Furthermore, Silicon quantum dot itself is a zero-dimensional (0D) structure. Hence obviously nc-Si:H/c-Si heterojunction is electrically a three dimensional (3D)-2D-0D system. The existence of the low-dimensional structure would result in the appearance of quantum effect at low temperatures. Current-voltage (I-V) curves at low temperatures directly exhibit the existence of resonant tunneling peak. It should be noted here that though many mature theories, such as Matveev-Larkin (ML) theory, have succeeded in explainng a lot of resonant tunneling phenomena in low dimensional systems, they focus on the demonstration of resonant tunneling between two given states and could not clearly explain the resonant tunneling process in our 3D-2D-0D system. We need to establish a resonant tunneling diode (RTD) model based on the quantum transport theory to explain the resonant tunneling phenomena observed in I-V curves of nc-Si:H/c-Si heterojunction.Within the framework of quantum transport theory, a RTD model based on the self-consistent potential calculation and transfer matrix procedure, has been constructed, in which a great number of factors, such as charge contribution, electron accumulation at the interface, and electron-electron interactions have been taken into account. The model has been used to simulate the resonant tunneling peak in a nc-Si:H(n)/c-Si(p) heterojunction with a doping ratio in nc-Si:H of 0.1 % and an acceptor concentration in c-Si of 1.0×1016 cm-3. The energy band parameters required in the simulation all come from either the literature or the experiments. The grain size and crystallinity of the silicon quantum dot are obtained from X ray diffraction and Raman shift experiments, and the conduction band discontinuity of the heterojunction is gained from the capacitance-voltage (C-V) measurement, while the barrier height and width are achieved from the literature. The theoretical calculation not only succeed in simulating the experimental results and illustrating the origin (3D-2D tunneling) of the tunneling peak, but also indicates the doped concentration of the heterojunction can control the resonant tunneling characteristics.By the aid of sequence tunneling theory, we have further showed the detailed control of the tunneling current by the position of 2D and/or 0D states. The calculated results show that by properly changing the doped concentration of the heterojunction, the position of 2D-0D tunneling peak would vary and evenmore appear/disappear. The double-peak resonant tunneling (3D-2D and 2D-0D) structure has been observed in the I-V curve of a nc-Si:H(n)/c-Si(p) heterojunction which has a doping ratio of 0.8 % and an acceptor concentration of 7.6×1014 cm-3. This above control of the resonant tunneling current by the doped concentration can provide the essential designing parameters for the RTD devices. We have also observed the periodic negative differential conductance peaks in the above nc-Si:H(n)/c-Si(p) heterojunction. The theoretical analysis indicates the phenomena originate from the accumulation and depletion of electrons tunneling through the nanodot layers in the neutral region, that is, the 0D-0D resonant tunneling occurring in the silicon quantum dots. Therefore, all the tunneling regimes related to the low dimensional states in the nc-Si:H/c-Si heterojunction have been explored.Furthermore, we have investigated the carrier transport characteristics of InN thin film grown on GaAs substrate by reactive frequency magnetron sputtering with different growth conditions. The experimental temperature-dependent conductance illustrates the conductance of the InN thin films increases with the decreasing temperature and tends to be a steady value at low temperatures. At low temperature the neutral impurity scattering plays the dominant role, which leads to the steady varying tendency of the conductance. We also found the grain boundary barrier model could explain the observed abnormal conductance behavior with the temperature from room temperature (RT) to low temperature (LT). The traps in the InN thin films can capture the free carrier and form the grain boundary barrier, which partly blocks the motion of the free carrier in the InN thin films. The grain boundary barrier height in the InN thin film is low due to the high trap concentration, eventually resulting in the abnormal conductance behaviour that conductance increases with the decreasing temperature.Not only can we use the grain boundary model to simulate and explain temperature-conductance characteristics, but also the variation of the barrier height with the bias. Both the experimental and self-consistent calculating results indicate that the barrier height decreases with the increasing bias, and finally disappears at larger bias. For the case of a pure ballistic transport, all the potential energy of the electrons is converted into kinetic energy. When kinetic energy is larger than threshold energy, electrons in the n-type InN films can impact-ionize a valence state and thereby create a hole. Holes created as minority carriers probably have to go back to the interface due to the force of the electric field. Part of the holes are trapped in the grain boundary during the diffusion; the accumulation of holes at the grain boundaries compensates parts of the negative screening charge, makes the charge in the grain boundary decrease, and consequently lowers the barrier height until the barrier disappears in the large applied bias. The relationship between the bias and barrier height illustrates the carrier transport properties in InN thin film is govern by the holes trapping at the grain boundary. We have also obtained the trap concentration according to the grain boundary barrier height, for the barrier forms after the traps at the grain boundary capture an amount of free carriers. The calculated trap concentration can be proved by the Micro-Raman experiments. All above facts confirm that the grain boundary barrier scattering play a dominant role in the InN thin film grown on GaAs substrate by rf magnetron sputtering in the range of temperature from RT to LT.In addition, we have still discussed the negative capacitance induced by the traps and charges at the InN/GaAs interface. A negative capacitance effect has been observed throughout the measured frequency in the InN thin film on GaAs substrate grown by rf magnetron sputtering. The coplanar electric contacts show that the negative capacitance probably derives from the contribution of InN thin film, or InN/GaAs interface, or sum of them. The direct comparison experiments between the InN thin films on GaAs and sapphire substrates clearly confirm that the NC effects of InN thin films on GaAs substrates partly comes from the interfaces between InN thin films and GaAs substrates. The barrier free model declares that the capacitance of the InN thin film is negligible in magnitude, as compared to the experimental data, and the negative capacitance effect originates from the contribution of InN/GaAs interface. There are a lot of traps existed at the InN/GaAs interface due to the larger lattice mismatch. In the presence of an ac perturbation with a small voltage, the interface traps can retain a sufficient quantity of charge so that they build a dipole layer, which modulates the barrier height. The variation of barrier height arises mainly from carrier capture and emission at interface states. This process requires a certain period of time, which makes the variation of barrier height lag behind small voltage and therefore results in the NC effect.Based on a transient current model of charging-discharging and inertial conducting for the InN/GaAs interface states, we have further investigated the InN/GaAs interface characteristics. This model can not only successfully simulate the experimental capacitance-frequency characteristics of the InN/GaAs interface, but also give some parameters related with the interface properties. The relationship between these interface-parameters and the growth conditions indicates that the interface characteristics are closely related with the growth condition and quality of the thin film. In another word, it shows the samples which have better quality of thin film have less interface traps and charges.This work is supported by the Natural Science Foundation of China under contract Nos. 10125416, and 10674094, Shanghai municipal projects of 03DJ14003, 05DJ14003, and 06JC14039, as well as the National Minister of Education Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) of IRT0524.