Electron Transport In Low–Dimensional III-V Semiconductors

Low dimensional III–V semiconductors such as quantum wells, quantum wires and quantum dots based on GaAs, InP materials are of great interest due to their promising future of applications in nanoelectronics. This dissertation details several selected topics on the electron transport in those materials and devices. The topics which we have studied include both fundamental physics and future applications. The main achievements are as follows:1. Self-assembled semiconductor quantum dots(1) Using molecular beam epitaxy technology, extremely low density self–assembled InAs/GaAs quantum dots grown in the Stranski–Krastanow mode are achieved. The sheet density of quantum dots has been reduced to around 2.5×107cm–2 by a combination technique of of post–annealing and wafer non–rotation. Those low density quantum dots are potentially useful in single electron nanodevices and single photon light sources. The growth mechanism of the quantum dots is also studied experimentally.(2) The intershell and hybridized energy states in InAs/GaAs quantum dots, which are many body effects, are observed. We change the excitation power during the photoluminescence study of the quantum dot sample, and have observed intershell and hybridized state signals of S', P', D', which have not been reported before.2. Quantum dot resonant tunneling diode Based on the growth method above, we fabricate vertical quantum dot resonant tunneling devices and measure their electronic properties.(1) In a Schottky diode structure, self–organized InAs/GaAs quantum dots are embedded into the intrinsic layer to study the resonant tunneling behavior. A 4nm AlAs layer is inserted to suppress the thermal current. Theory based on the analytical solution of the Poisson's equation has been developed, which is in good agreement with the experimental findings. The potential profiles of the device during resonance are calculated. At 77K, electron resonant tunneling through quantum dots under both positive and negative bias (0.4V and–0.8V) is observed in the I–V curve.(2) We fabricate a symmetric double barrier InAs/AlAs quantum dot resonant tunneling diode. The optical properties of the quantum dots are characterized by photoluminescence and the morphology of the quantum dots are observed by field–emission scanning electron microscopy while selectively etching away the capping layer. At both 77K and room temperature, the I–V and C–V curves of the devices are studied carefully. Electron resonant tunneling effect and the hot electron charging/discharging effect are observed at 77K. The resonance with the ground or first excited states of the quantum dots can be observed even at room temperature, which is very important in terms of future applications. A simple model including the In segregation effect is introduced to explain the experimental findings.3. Three terminal ballistic junctions(1) We fabricate three terminal ballistic junctions by electron beam lithography and wet etching on a two–dimensional electron gas material grown by metal–organic vapor phase epitaxy. They are a kind of room temperature nonlinear nanoelectronic devices. In the push–pull measurement, the voltage output from the central branch is always negative.(2) Using the above-mentioned nonlinearity, the room temperature frequency mixing and phase detection functionality is realized in a single three terminal ballistic junction. In the phase detector, compared to the static case, there is almost no output decay at radio frequency.4. Integrated nano RS flip–flop(1) In a single quantum wire transistor, the I–V output curve and the transfer properties are studied. It is very like the commercial normally–on field effect transistor except that at positive gate voltage the gating efficiency sharply goes down. Based on two quantum wire transistors, integrated nanoelectronic RS flip–flop can be realized. Two kinds of RS flip–flop circuit design are studied on the same device. In the second design, the inputs of the device are shifted to negative voltage region and the device stability is much more improved. A logic swing gain of as large as 4 times is achieved.(2) We fabricate a logic inverter using a ballistic junction and a side gate. Under certain conditions, this device can be used as the interface circuit of TTL and CMOS. Based on that, a new integrated nanostructure composed of two three terminal ballistic junctions and two additional in–plane gates is prepared. Two kinds of RS flip–flop circuit are measured at room temperature. In the second design, the inputs of the device are changed to negative voltage regime and the device is much more stable. We have very large noise margins at both high and low level logic inputs (VNH=VNL=0.4V), which is essential for further device integration. In regard to those results, three terminal ballistic junctions and quantum wire transistors can serve as the building blocks in future nanoelectronics.