Font Size: a A A

The Transport Properites Of Novel Semiconductor Materials And Infrared Devices

Posted on:2009-12-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:H B YeFull Text:PDF
GTID:1118360242495156Subject:Condensed matter physics
Abstract/Summary:PDF Full Text Request
By the aid of magnetic-field-dependent Hall effect measurements combined with the mobility spectrum analysis (MSA) and Shubnikov-de Hass (SdH) measurements, we have investigated the electrical transport properties of the p-type ZnO thin films and three kinds of novel semiconductor infrared devices, including GaAs far-infrared (FIR) mirror structures, InAsSb quantum dots (QDs) mid-infrared light emitting diodes (LEDs) and InAs/GaSb/AlSb quantum cascade lasers (QCLs) material.Usually, due to the self-compensation effect from native defects and the low solubility of the acceptor dopants in ZnO, high acceptor doping seemly becomes necessary to obtain p-ZnO, which will result in the low hole mobility. Through temperature-dependent Hall effect measurements, we have achieved the high mobility and conductivity in our studied N-In codoped p-type ZnO thin films. As the average grain size of our polycrystalline p-type ZnO thin films is among 8-10 nm, the bands will be effectively flat. Due to the lack of the grain boundary barrier effect and weak ionized impurity scattering, relatively high mobility can be achieved in our polycrystalline p-ZnO thin films. Meanwhile, we have observed the thermal activating behavior of the hole concentration and the yielded activation energy (~20 meV) is related to the shallow grain boundary trapping level rather than the acceptor level. We have also revealed the scattering and conduction mechanisms in these p-ZnO thin films. As we know, Hall effect measurements will become difficult and unreliability when the measured ZnO samples exhibit high resistivity. Hence, we have established an optical method to extract the useful carrier transport information from the Raman spectrum analysis. The good agreement between Raman and Hall measurements on the transport information demonstrates the reliability of this method and reconfirms the obtained high mobility in our p-ZnO thin films.It should be noted that the carriers with different energy will be scattered differently in the transport process, which results in the statistical distribution of carrier velocity. Under the relaxation time approximation at weak electric field, the statistical distribution of carrier velocity will be considered as the mobility distribution of carrier concentration. However, the dependency of relaxation times on energy is neglected in the classic Hall effect measurements under a single magnetic field and cannot be employed for obtaining the individual transport information of multi-carrier in measured samples. Fortunately, the magnetic-field-dependent Hall effect measurements have the ability to extract the transport parameters of all carrier species present within samples that are contributing to the conduction process by using the MSA, based on the distribution of relaxation times. The MSA transforms the experimental Hall data into the dependence of the conductivity density function on mobility, in which each carrier contributing to the total conductivity appears as a separate peak at a given mobility.In order to reveal the carrier transport properties and different doping mechanisms, we have presented a compared investigation of N-In codoped and N-doped p-type ZnO thin films on the basis of the magnetic-field-dependent Hall effect measurements followed by the MSA. The transport information of three carrier species has been obtained, including the free electrons and holes from ZnO thin film as well as the two-dimensional hole gas (2DHG) from the interface between ZnO film and intrinsic silicon substrate. The yielded mobilities of free electrons and holes at room temperature are both larger than 100 cm2/Vs. When the conductivity contributions between free electrons and free holes are comparable, the existence of high mobility 2DHG becomes especially important and results in the high mobility and conductivity of p-ZnO thin films grown on intrinsic silicon substrates. Through the investigation of temperature-dependent photoluminescence spectra, we have clarified that the N-In codoping, compared with the N-doping, leads to the decrement of acceptor binding energy and increment of donor binding energy in ZnO, and also broadens the acceptor level. This result is consistent with the observation of more free hole concentration in the mobility spectra for N-In codoped ZnO. Meanwhile, the introducing of In dopants results in the more free electron concentration in N-In codoped ZnO. In addition, we have revealed the different carrier recombination processes in N-In codoped and N-doped p-type ZnO thin films, as well as the mechanism of the deep-level visible emission (~2.5 eV) in undoped ZnO.Secondly, we investigate the carrier transport and optical properties in doped/undoped GaAs single-period FIR mirror structures for GaAs-based FIR device application. Through variable magnetic field Hall and SdH measurements, the carrier concentration, mobility and scattering times are obtained and analyzed. It is found that ionized impurity scattering is the dominant scattering mechanism at low temperatures for the GaAs FIR mirror structures. The calculation of energy flux along the depth of the mirror structures shows that most of the absorption falls inside the highly doped bottom GaAs layer. Even though the reflection is lower than the traditional distributed Bragg reflectors (DBRs) working in the near- and mid-infrared, the present GaAs single-period FIR mirror structures show perfect enhancement effect in a wide FIR region compared with a single doped GaAs layer. The calculated FIR transmission and reflection spectra agree well with the experimental results for the mirror structures, demonstrating that all the optical analysis about GaAs FIR mirror structures is reliable.Thirdly, we investigate the electrical transport properties of InAs1-xSbx QDs mid-infrared LEDs under magnetic field. With relatively small Sb content (x=0.2), the InAs1-xSbx sample exhibits quite a smooth continues strained layer. Coalesced QDs and isolated QDs are formed with increasing Sb composition (x=0.2-0.3) and hence strain, while our studied InAs1-xSbx sample is just at the critical composition (x=0.25). The transport information of three carrier species has been obtained through magnetic-field-dependent Hall measurements followed by the MSA, including heavy holes and light holes from coalesced InAsSb QDs, as well as two-dimensional electron gas (2DEG) from InAs1-xSbx strained layer. The major scattering mechanism for heavy holes is ionized impurity scattering. While alloy scattering limits the low temperature mobility and photon scattering limits the high temperature mobility for light holes. The 2DEG mobility exhibits temperature-independent at low temperatures and decreases with temperature due to the photon scattering limits at high temperatures. We have also analyzed the effective mass and quantum scattering time of 2DEG by using SdH measurements.Finally, we have studied the carrier transport phenomenon in InAs/GaSb/AlSb QCLs multilayer structures. The transport information of electrons in GaSb cladding layer and 2DEG from InAs quantm wells have been extracted and analyzed. The electron mobility in GaSb exhibits the characteristic behavior of bulk carrier. While the 2DEG mobility is much smaller than the electron mobility in InAs film because of the block off effect from multi-periods of AlSb barrier layer. With the decrease of temperature, the mobility peak of 2DEG coincides with the peak of GaSb electrons and its conductivity contribution can be ignored at very low temperatures due to the carrier freeze-out behavior. Therefore we have observed the SdH oscillations mainly from GaSb electrons and calculated effective mass and quantum scattering time. Through the ratio between classical scattering time and quantum scattering time, we can identify interface roughness scattering as the dominant scattering mechanism at low temperatures for GaSb electrons.This work is supported in part by the Natural Science Foundation of China under Contract Nos. 10125416, 60576067, 10674094, and 10734020, National Major Basic Research Project of 2006CB921507, and Innovative Research Team in University (PCSIRT) of IRT0524.
Keywords/Search Tags:p-type ZnO thin films, novel semiconductor infrared devices, magnetic-field-dependent Hall effect measurements, mobility spectrum analysis, Shubnikov-de Hass oscillations
PDF Full Text Request
Related items