Optical Properties Of IV-VI Semiconductor Low-Dimensional Structures

Ⅳ-Ⅵsemiconductors such as PbSe and PbTe have unique intrinsic characteristics including narrow and direct band gap (-0.3eV), symmetric band structure, absence of a degenerate heavy hole band, and low Auger recombination rates, etc. These characteristics make them as promising materials for the development of mid-infrared optoelectronic devices, which can be widely used in a variaty of applications such as ultrahigh-sensitivity gas trace analysis, atmospheric pollution monitoring, infrared imaging, biology and medical diagnosing. Due to the quantum confinement, theⅣ-Ⅵlow dimensional structures (quantum wells and quantum dots) exhibit changed density of states, narrower spectra of optical transitions, and more effective interaction between photons and electrons as compared to the bulk materials. The band extrema ofⅣ-Ⅵsemiconductors are at the four equivalent L points of the Brillouin zone. The constant energy surfaces are prolate ellipsoids of revolution characterized by the longitudinal and transverse effective masses ml* and ml*, respectively. The major axes of the ellipsoids are in the [111] direction. These intrinsic characteristics are much different from more commonly studiedⅢ-ⅤandⅡ-Ⅵsemiconductors to justify further experimental and theoretical investigation for the optical transition and optoelectronic devices application ofⅣ-Ⅵlow dimensional structures.The thesis is divided into two parts:the theoretical simulation of optical transitions and the molecular beam epitaxy experiments. A theoretical model based on the four-band k·p envelope function approach was developed to calculate electronic band structures inⅣ-Ⅵlow dimensional structures such as QWs and QDs with different growth orientations. The validity and utility of the model was proof-test through simulating the room temperature photoluminescence spectra measured from PbTe/CdTe QWs and QDs. The Rashba spin-oribt-splitting inⅣ-Ⅵsemiconductor asymmetric QWs vs growth orientation, well width, temperature and kx was studied. The experiments were conducted to grow PbSe and PbTe single crystal thin films and their QDs by self-designed molecular beam epitaxy equipments. The PbSe QDs grown on PbTe buffer layer display room temperature photoluminescence (PL) at wavelength of 3.8μm, which was analyzed theoretically. The main results are as follows: 1. A four-band k·p model was developed that takes into account the anisotropic and nonparabolic band structure ofⅣ-Ⅵsemiconductors. The model can be used to describe interband optical transitions inⅣ-ⅥQWs and QDs with various orientations. The Rashba spin-orbit coupling termαso·σ'×k⊥in every energy valley of low dimensional structures was derived.2. The theoretical model developed for QW structures was applied to calculate spontaneous emission spectra of PbTe/CdTe (001) single quantum well structure, and the calculated results are in agreement with the PL spectra measured from experiments. For PbTe/CdTe quantum wells (QWs) with well width of 20 nm, two emission peaks are dominant in the spontaneous emission spectra, which blueshift with the increase of carrier density while intraband relaxation is considered in calculation. The shift of ground state emission is from 372 meV to 397 meV and that of the first excited state emission is 15 meV, as carrier density increases from 2×1017 to 2.8×1018cm-3. The shifts of transition energies are attributed to band filling effect. Further results show that [110]Ⅳ-ⅥQWs has the lowest injected carrier concentration to produce optical gain than QWs with other growth orientations, which is an advantage for mid-infrared QW laser operating at room temperature.3. The QD theoretical model developed was applied to simulate spontaneous emission spectra of PbTe/CdTe QDs. The simulation results show that two types of quantum structures were formed by the thermal annealing of the PbTe/CdTe quantum wells:the high symmetric cubic QDs (typical size is 25nm) and elongated QDs (average height is 8 nm) and the calculations are in good agreement with the experimental observations of the dot sizes and photoluminescence. The optical gain calculation of PbTe QDs suggests PbTe/CdTe quantum dots with dot size of 15-20 nm are promising materials for mid-infrared lasers, which may produce optical gain higher than 5000 cm-1 as injection carrier density ranges from 0.3 to 3×1018cm-3.4. The Rashba spin-orbit-split inⅣ-Ⅵasymmetric QWs as function of growth orientation, well width, temperature and kx was investigated theoretically. The calculated results show that Rashba spin-orbit-split in these structures decreases with temperature increasing and the magnitude can be larger than 1 meV, which is sensitive to the structure parameters. As the structure parameters are the same except orientation, the well widths at which the Rashba splitting reach the maximum are different:[100]QWs is 4.9 nm, [111]QWs is 2.1 nm for L-valley and 5.6 nm for O-valley, [110]QWs is 3.8 nm for O1-Valley and 5.9 nm O2-Valley. Large and sensitive Rashba spin-orbit coupling may makeⅣ-Ⅵasymmetric QWs as promising materials for spintronic devices.5. PbSe and PbTe single crystal films and their QDs were grown on BaF2 (111) substrate by self-designed molecular beam epitaxy system. Evolution of PbSe surface morphologies with Se/PbSe beam flux ratio (Rf) has been studied by atomic force microscopy (AFM) and high-resolution X-ray diffraction. Growth spirals with monolayer steps on PbSe surface are obtained using high beam flux ratio, Rf≥0.6. As Rf decreases to 0.3, nano-scale triangle pits are formed on the surface and the surface of PbSe film changes to 3D islands when Rf=0. Glide of threading dislocations in<110>{100}-glide system and Pb-rich atom agglomerations are the formation mechanism of spiral steps and trangle pits. The PbSe QDs grown on PbTe buffer layer surface characterized by triangle pits display room temperature PL spectra and theoretical analyse is applied to the PL spectra.