Electronic properties of multi-quantum dot structures in Cd 1-xZn xS alloy semiconductors

In this paper, we present a theoretical study of the quantized electronic states in Cd_1-xZn_xS quantum dots. The shape of the confining potential, the subband energies and their eigen envelope wave functions are calculated by solving a one-dimensional Schr?dinger equation. Electrons and holes are assumed to be confined in dots having a flattened cylindrical geometry with a finite barrier height at the boundary. Optical absorption measurements are used to fit the bandgap edge of the Cd_1-xZn_xS nanocrystals. An analysis of the electron band parameters has been made as a function of Zn composition. Two main features were revealed: (i) a multiplicity in Cd_1-xZn_xS quantum dots with different crystalline sizes has been found to fit accurately experimental data in the composition range 0 ≤x ≤0.2; (ii) the fit did not, however, show a multiplicity for x higher than 0.4. On the other hand, we have calculated the energy level structure of coupled Cd_1-xZn_xS semiconductor quantum dots using the tight-binding approximation. As is found the Zn composition x = 0.4 is expected to be the most favorable to give rise a superlattice behavior for the Cd_1-xZn_xS quantum dots studied.     

Optical investigations of ZnxCd1-xS nanostructures

Zn_xCd_1-xS nanostructures with (x = 0, 0.25, 0.5, 0.75, 1) have been grown on glass substrates using spray pyrolysis technique. X-ray diffraction results have showed that Zn_xCd_1-xS nanostructures were formed with hexagonal and cubic structures. The structural parameters have been evaluated as a function of concentration (x). Also, the optical properties that depend on the mole fraction (x) are investigated for Zn_xCd_1-xS nanostructures.     

First-principles study of structrural and corrected band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO systems

A first-principles method based on density functional theory(DFT),a generalized gradient approximation(GGA),and a projector-augmented wave(PAW) are used to study the structual and band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO(0 ≤x≤1) ternary alloys.By taking into account all of the possible structures,the band gaps of Zn1-xCdxO and Zn1-xMgxO alloys are corrected and compared with experimental data.     

Comparison study of energy bands and Wannier-Mott excitons in mixed Zn(P As ) and Zn Cd P crystals

Excitonic absorption, reflection and photoluminescence spectra of mixed Zn(P_1-xAs_x)₂ crystals over the full range of x ( 0 ? x ? 1) and Zn_1-xCd_xP₂ crystals at 0 ? x ? 0.05 have been studied at low temperatures (1.8 K). The decrease of the energy gap in Zn(P_1-xAs_x)₂ at the increase of x occurs slightly sublinearly. The rydbergs of excitonic series in this crystals decrease as well, and the dependences Ry ( x ) for all series are strongly superlinear at small x. In Zn_1-xCd_xP₂ crystals the energy gap and rydbergs decrease at the increase of x (at 0 ? x ? 0.05) as well. The dependences of E_g and Ry on x are considerably stronger in Zn(P_1-xAs_x)₂ than in Zn_1-xCd_xP₂. At the increase of x the half-width of excitonic absorption lines increases monotonically in both type crystals that is evidence of the increasing role of fluctuations of crystal potential. Received 13 March 2002 Published online 9 July 2002     

Effects of Cd compensation on the photoluminescence of Cd1-xZnxTe single crystals grown by the modified Bridgman method

In order to deal with the phenomenon of Cd evaporation during the growth of Cd_1-xZn_xTe (x=0.1) crystals, Cd compensation was adopted during the growth by adding excess Cd into the raw materials. Photoluminescence (PL) spectra were used to investigate the effects of Cd compensation on the properties of Cd_1-xZn_xTe. A free exciton (FE) peak appeared in the near band-edge region after Cd compensation, which indicated that the concentration of Cd vacancies (V_Cd) was reduced in Cd_1-xZn_xTe crystals by Cd compensation. The donor–acceptor pair (DAP) peak became dominant in the PL spectrum and its first and second order phonon replica could also be easily identified after Cd compensation, which was only a weak hump in the case of Cd_1-xZn_xTe crystals without Cd compensation. It possibly meant that impurities of Al and In were released from the V_Cd-related complexes. In addition, the deep energy level transition D peak, decreased obviously after Cd compensation, which confirmed that Cd compensation could reduce the dislocation density effectively. PACS 71.20.Nr; 71.55.Gs; 78.55.-m     

Investigation on the structural properties and correlation of the optical constants of Cd1-xZnxS thin films with bath deposition temperature

Cd_1-xZn_xS thin films were deposited by chemical bath deposition (CBD) technique, which is simple and cost effective, in a chemical bath containing appropriate amount of cadmium acetate, zinc acetate, and thiourea as precursors, in a clean glass substrate. The deposition was carried out by varying the bath temperatures (70 °C, 75 °C, 80 °C, and 85 °C) of the precursor solution. The XRD results indicate the existence of hexagonal structures of Cd_1-xZn_xS with an average crystallite size of ∼ 27–41 nm. EDX studies confirm the presence of Cd, Zn, and S in the films. HRTEM and SAED patterns show the crystalline nature of the films with the coexistence of the hexagonal phase. The optical constants viz; optical band gap, Urbach energy, static refractive index, and optical conductivity were studied by using UV- Vis transmission spectra as a function of CBD temperature. It was observed that with the increase of bath temperature in the above range, there were concomitant decreases in optical band gap from ∼3.3 to 2.8 eV. The Urbach energy, optical conductivity, and static refractive index of the films increase with the increase in bath deposition temperature. FTIR studies confirm the formation of ternary Cd_1-xZn_xS thin films.     

Photoluminescent analysis of Zn1-xCdxO alloys

The photoluminescent (PL) spectra of Zn_1-xCd_xO (0≤x≤0.53) alloy films were obtained successfully. A new explanation from the viewpoint of band structure is brought forward to comprehend the PL nature of the alloy films. According to this explanation, the near-band-energy emissions of the Zn_1-xCd_xO (x>0) films are caused by the radiative transitions between the Zn4s–Cd5s hybrid level and the O2p level, and the broadenings of the two levels are responsible for the gradually increased line width of the PL peak of the film; Zn3d and Cd4d orbital levels have great effects on the band-gap variations of the alloys. In addition, a quadratic equation is put forward to depict the relationship between the band gaps E_g of the alloys and their Cd contents x, i.e. E_g(x)=3.30-1.22x+1.26x² (0≤x≤0.53). PACS 78.55.-m; 78.55.Et; 81.15.Cd     

Lattice strain and p-d repulsion affecting electronic structure of wurtzite Zn1-xCdxO alloys

In the paper, density of states, band structure and electron density difference of Zn1-xCdxO are calculated by first principles, here x varies from 0 to 0.75 at intervals of 0.125, and the band gap obtained from band structure changes from 0.968 eV to 0.043 eV. The lattice strain and p-d repulsion theory are used to investigate variation of the band gap, the results obtained show that the variation is mainly due to the lattice tensile strain. The p-d repulsion in Zn1-xCdxO cannot be neglected. In addition, electron density difference can be used to verify the results.     

Photoluminescence measurements in the phase transition region of Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S films

Thin films of Zn_1−x Cd _x S (0.1 ≤ x ≤ 0.5) were prepared by using pulsed laser ablation technique on corning glass substrates. Phase transition from cubic to hexagonal in Zn_1−x Cd _x S films is determined by X-ray diffraction analysis. We observed a lowering in the phase transition temperature with increase in the cadmium concentration. Transmission electron microscopy suggests the crystalline nature of thin films with average particle size of 15 nm. The grown Zn_1−x Cd _x S samples show the high peak intensity ratio of the near band edge emission to the defect center luminescence even at room temperature, which indicates the small concentration of complex defects in the samples. Photoluminescence measurement show stoichiometric dependence of the energy band gap and is found to have quadratic dependence on x.     

Zn1-xMnxSe的低温磁化率和自旋玻璃现象

用超导量子干涉器(SQUID)磁强计对稀释磁性半导体Zn_1-xMn_xSe(0.1≤x≤0.50)的低温低场直流磁化率作了测量,测量温度从4.2K到30K,测量磁场为15Oe。当x≥0.30时,从磁化率-温度曲线的浑圆峰值,观察到了自旋玻璃的转变。自旋玻璃的转变温度T_f,对x=0.30,0.40,0.50,分别为10.5K,16K,19.5K。给出了顺磁相和自旋玻璃相的相图。比较了Zn_1-xMn_xSe和Cd_1-xMn_xSe的自旋玻璃转变温度,发现对同样的Mn离子浓度,Zn_1-xMn_xSe的T_f高于Cd_1-xMn_xSe的T_f,用交换作用的理论作了讨论。 关键词：     

Microwave-assisted synthesis of Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S–MWCNT heterostructures and their photocatalytic properties

The multi-walled carbon nanotubes (MWCNTs) wrapped with hexagonal wurtzite Zn _x Cd_1−x S nanoparticles with a uniform and small diameter have been prepared to form Zn _x Cd_1−x S–MWCNT heterostructures by microwave-assisted route using Zn(Ac)₂, Cd(NO₃)₂, and thioacetamide as the reactants_. The heterostructures have been characterized by X-ray powder diffraction, scanning and transmission electron microscopy, high-resolution transmission electron microscopy, photoluminescence (PL) and PL excited lifetime. Despite the analogous size and configuration, the Zn _x Cd_1−x S–MWCNT (x = 0, 0.2, 0.5, 0.8, 1) with different Zn concentration exhibit composition-dependent absorption properties in the visible zone. The PL peak positions of Zn _x Cd_1−x S–MWCNT change gradually from ZnS–MWCNT to CdS–MWCNT. The Zn _x Cd_1−x S–MWCNT shows different photocatalytic activity towards the photodegradation of fuchsin acid under visible light illumination, photocatalytic activity of the Zn _x Cd_1−x S–MWCNT decreases gradually with the increase in the Zn concentration, the Zn_0.2Cd_0.8S–MWCNT possessed the best photocatalytic activity. After recycling thrice, the photocatalysts still have about 85% efficiency.     

Structure, refractive-index dispersion and the optical absorption edge of chemically deposited ZnxCd(1-x)S thin films

Zinc cadmium sulfide, Zn_xCd_(1-x)S, thin films have been deposited by a simple and inexpensive chemical bath deposition method from an aqueous medium using thiourea as a sulfide-ion source. The structure of the deposited films has been characterized by X-ray diffraction and transmission electron microscopy. It was observed from X-ray diffraction that the as-deposited films were amorphous in nature. However Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h show a crystalline structure with a small scattering volume. The obtained results were confirmed throughout the transmission electron microscopy and the corresponding electron-diffraction patterns. The optical constants of Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h in the compositional range 0≤x≤1 were estimated using transmission and reflection spectra in the wavelength range 300–2500 nm. The band gap varies non-linearly with the value of x. The dependence of the refractive index on the wavelength obeys the single-oscillation model, from which the dispersion parameters and the high-frequency dielectric constant were determined. A graphical representation of the surface and volume energy-loss functions was also given. Received: 23 February 2001 / Accepted: 26 February 2001 / Published online: 27 June 2001     

The optical properties of CdxZn1−xS thin films on glass substrate prepared by spray pyrolysis method

A series of Cd_xZn_1−xS thin films have been deposited on glass substrates using spray pyrolysis technique. The crystallinity and microstructure of Cd_xZn_1−xS thin films have been investigated by X-ray diffraction (XRD). Based on the results of Hall measurements, the films obtained were an n-type semiconductor. The X-ray data analysis of Cd_xZn_1−xS thin films showed that the grain size of the Cd_xZn_1−xS increased with increase in Cd composition. It is observed that the band gap increases as the Cd composition decreases. The results also showed a blue shift of absorption edge of optical transmission spectra is increases as Zn ratio increases. The effects of Cd composition on the structural and optical properties of Cd_xZn_1−xS thin films were related to their grain size, stress and carrier concentration.     

Electrical and photovoltaic properties of Cd1? x Zn x S/ p -GaAs heterojunctions

Cd_1−x Zn _x S/p-GaAs heterojunctions for solar cell applications have been prepared by growing single crystal Cd_1−x Zn _x S epitaxial layers on (111)GaAs substrates through a vapour phase chemical transport method using the close-spaced geometry and H₂ as a transport agent. Electrical and photovoltaic properties of the heterojunctions have been investigated and discussed in connection with the main features of the growth technique. AM1 power conversion efficiencies up to 6.2% have been measured and possible improvements have been examined.     

First-principles study of the structural,electronic and optical properties of the cubic triangular quaternary ZnxCdyHg1-x-yTe alloys under hydrostatic pressure

In the present computational study, we have explored the structural, electronic and optical properties of ZnTe, CdTe and HgTe binary compounds and their ternary alloys Zn_xCd_1-xTe, Zn_xHg_1-xTe and Cd_xHg_1-xTe as well as their ordered quaternary Zn_xCd_yHg_1-x-yTe alloys using the full potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory. We have numerically estimated the total energies, the lattice parameters, the bulk moduli and their first pressure derivative using the generalized gradient approximation (GGA). The band structure is computed using the modified Becke-Johnson (TB-mBJ) approximation. Results of our study show a nonlinear dependence of the composition on the lattice constant, bulk modulus and band gap for the binary and ternary compounds as well as for the quaternary alloys. Additionally, the dielectric function, the refractive index and the loss energy were also reported. The pressure effect on the band gap energy and optical properties were also investigated and reported. Our results are in good agreement with experimental values and theoretical data available in the literature.     

Infrared lattice-reflection spectroscopy of Zn1−x CdxSe epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Results are presented of the first measurements of infrared reflection spectra of Zn_1−x Cd_xSe films (x=0–0.55; 1) grown on a GaAs substrate by molecular-beam epitaxy. It is shown by a mathematical analysis of the experimental spectra that the investigated Zn_1−x Cd_xSe alloy system manifests a unimodal rearrangement of its vibrational spectrum as the composition is varied. Fiz. Tverd. Tela (St. Petersburg) 41, 982–985 (June 1999)     

Cathodoluminescence spectra of Cd1−x Zn x Te solid solutions

An investigation is made of the cathodoluminescence, electrical properties, and structural features of crystals of Cd_1−xZn_xTe solid solutions with composition x ranging from 0.02 to 0.20 mole fraction as compared with CdTe and ZnTe. The main bands of the cathodoluminescence spectra are identified within the range 500–2700 nm. The role of oxygen in Cd_1−xZn_xTe cathodoluminescence is shown. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 96–100, January–February, 2000.     

The optical properties of MgxZnl-xO thin films

Mg_xZn_1-xO thin films have been prepared on silicon substrates by radio frequency magnetron sputtering at 60℃. The thin films have hexagonal wurtzite single-phase structure and a preferred orientation with the c-axis perpendicular to the substrates. The refractive indices of Mg_xZn_1-xO films are studied at room temperature by spectroscopic ellipsometry over the wavelength range of 400--760\,nm at the incident angle of 70℃. Both absorption coefficients and optical band gaps of Mg_xZn_1-xO films are determined by the transmittance spectra. While Mg content is increasing, the absorption edges of Mg_xZn_1-xO films shift to higher energies and band gaps linearly increase from 3.24.eV at x=0 to 3.90\,eV at x=0.30. These results provide important information for the design and modelling of ZnO/ Mg_xZn_1-xO heterostructure optoelectronic devices.     

白宝石衬底上生长的MgxZn1-xO晶体薄膜的结构和光学性能

在白宝石（sapphire）衬底上低温外延生长出了Mg_xZn_1-xO晶体薄膜.x射线衍射（XRD）及能量色散x射线（EDX）分析表明，Mg_xZn_1-xO薄膜的晶体结构依赖于薄膜中Mg的组分x，随着Mg组分的增大，Mg_xZn_1-xO薄膜的结构从与ZnO晶体一致的六方结构转变为与MgO晶体一致的立方结构.对Mg_xZn_1-xO薄膜的紫外透 关键词： 电子束蒸发反应 xZn_1-xO晶体薄膜')" href="#">Mg_xZn_1-xO晶体薄膜 结构和光学性能     

A comparison between optically active CdZnSe/ZnSe and CdZnSe/ZnBeSe self-assembled quantum dots: effect of beryllium

The composition and size of optically active Cd_xZn_1−xSe/ZnSe quantum dots are estimated with a previously developed method. The results are then compared with those obtained for Cd_xZn_1−xSe/Zn_0.97Be_0.03Se QDs. We show that introducing Be into the barrier material enhances both Cd composition and quantum size effect of optically active quantum dots.