Optical properties of undoped and iodine-doped CdTe

A comprehensive study of the properties of undoped and iodine-doped CdTe structures by photoluminescence (PL) and photoreflectance (PR) is reported. Undoped bulk CdTe and iodine-doped CdTe layers grown by metalorganic molecular beam epitaxy on (lOO)-oriented CdTe and (211)B-oriented GaAs substrates with electron concentrations ranging from 10¹⁴ to mid-10¹⁸ cm^-3 were included in this study. Lineshape modeling of 80KPL and PR spectra indicated the presence of both free exciton and donor-hole transitions at the higher doping levels. Strong PL and PR signals were also observed at room temperature. If only a single transition is considered for the analysis of the 300K spectra, the PL emission peak and the PR transition energy both exhibit a strong dependence on electron concentration for doped layers. However, lineshape modeling of the room-temperature spectra indicated the presence of multiple transitions consisting of free exciton and direct band-to-band transitions. The use of two transitions resulted in a constant value of bandgap over the entire range of conductivities studied. A strong correlation remained between the broadening of the PR and PL spectra and excess carrier concentration N_D-N_A. In addition, the E₁ transition energy measured by PR was found to vary dramatically with growth conditions.     

应变纤锌矿ZnO/MgxZn1-xO量子点中离子施主束缚激子的光学性质

Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong built-in electric field(BEF) in strained wurtzite Zn O/Mg0:25Zn0:75O quantum dots(QDs), the optical properties of ionized donor-bound excitons(D+, X)are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of(D+, X) complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of(D+, X) complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth.     

Photoluminescence measurements on undoped CdZnTe grown by the high-pressure bridgman method

The low temperature photoluminescence of Cd_0.91Zn_0.09Te grown by the high-pressure Bridgman (HPB) method exhibits a neutral donor bound exciton emission (D⁰X) at 1.65603 eV with its excited state (D⁰X^*) at 1.65798 eV and neutral acceptor bound exciton emissions (A⁰X) at 1.64566 eV and 1.65201 eV. Assuming a direct generation and subsequent relaxation of excitons at the D⁰X^* state, we demonstrate that the temporal evolution of the above emission bands is well reproduced by a set of rate equations. The resultant radiative-lifetime of 1.4 ns for the D⁰X and 1.5 and 2.0 ns for the A⁰Xs are compared with various CdZnTe's (CZTs) grown by the other methods to demonstrate the particular nature of the HPB CZT.     

Photoluminescence and raman studies of high quality CdTe:I Epilayers

Low-temperature photoluminescence (PL) studies of iodine-doped CdTe epilayers have been performed. A compensating acceptor center which gives rise to deep-level PL emission at 1.491 eV is identified. From selective excitation PL studies, we assign this 1.491 eV line to the recombination of an associate donor-acceptor close pair, consisting of nearest neighbor substitutional sodium and iodine atoms (Na_Cd-I_Te). This neutral defect complex has a localized mode of 36.5 meV, which is much larger than the bulk CdTe lattice mode of 21.3 meV. The electronic energy level associated with this defect is 115 meV below the conduction band. Also, we use a combination of selective excitation PL and Raman spectroscopies to determine the ionization energy of the isolated shallow iodine donor (I_Te) in CdTe. We find that the donor binding energy of this anion-site hydrogenic donor is 15.0 (±0.2) meV.     

Al_x Ga_(1-x)AS:Te的光致发光和DLTS研究(英文)

Photoluminescence (PL) and DLTS data are presented for Te-doped AlxGa1-xAs crystals in which carrier concentrations at 300K vary between 1 ×1016 and 1×1017cm-8 and compositions cover the range of 0.1相似文献   

非掺杂InP的低温光致发光研究

研究了2K低温下非有意掺杂InP单晶的光致发光谱,对近带边的辐射跃迁进行了仔细分析,报道了InP单晶在大功率光激发时束缚于中性施主的激子跃迁发光相对减弱、而束缚于中性受主的激子跃迁发光相对增强的现象,并探讨了其机制,确认了材料中存在Mg、Zn等残余受主杂质,并计算得到Mg受主的离化能为41.5meV.     

Exciton recombination in δ-doped type-II GaAs/AlAs superlattices

Radiative recombination of excitons in δ-doped type-II GaAs/AlAs superlattices (SLs) is studied experimentally. With an increase in the impurity density in δ-layers from 2×10¹⁰ to 7.5×10¹¹ cm^?2, the integrated intensity of SL photoluminescence (PL) decreases by a factor of 4–6; the intensity of excitonic PL drops considerably (up to 70–80 times), which is accompanied by an increase in the exciton radiative decay rate. Uniform doping of the SL does not result in the exciton PL quenching. Analysis of the temperature dependence and the kinetics of the PL indicate that impurity quenching of the excitonic PL in δ-doped structures is not related to a reduction in the exciton localization energy and cannot be explained by an increase in the density of nonradiative recombination centers. We conclude that the PL quenching is mainly caused by the appearance of built-in electric fields originating from ionized impurities, which hinders the formation of the excitons.     

Emissions and encapsulation of cadmium in CdTe PV modules during fires

Fires in residential and commercial properties are not uncommon. If such fires involve the roof, photovoltaic arrays mounted on the roof will be exposed to the flames. The amount of cadmium that can be released in fires involving CdTe PV and the magnitude of associated health risks has been debated. The current study aims in delineating this issue. Previous thermogravimetric studies of CdTe, involved pure CdTe and single‐glass PV modules. The current study is based on glass–glass CdTe PV modules which are the only ones in the market. Pieces of commercial CdTe photovoltaic (PV) modules, sizes 25×3 cm, were heated to temperatures up to 1100°C to simulate exposure to residential and commercial building fires. The temperature rate and duration in these experiments were defined according to standard protocols. Four different types of analysis were performed to investigate emissions and redistribution of elements in the matrix of heated CdTe PV modules: (1) measurements of sample weight loss as a function of temperature; (2) analyses of Cd and Te in the gaseous emissions; (3) Cd distribution in the heated glass using synchrotron X‐ray fluorescence microprobe analysis; and (4) chemical analysis for Cd and Te in the acid‐digested glass. These experiments showed that almost all (i.e., 99·5%) of the cadmium content of CdTe PV modules was encapsulated in the molten glass matrix; a small amount of Cd escaped from the perimeter of the samples before the two sheets of glass melted together. Adjusting for this loss in full‐size modules, results in 99·96% retention of Cd. Multiplying this with the probability of occurrence for residential fires in wood‐frame houses in the US (e.g., 10⁻⁴), results in emissions of 0·06 mg/GWh; the probability of sustained fires and subsequent emissions in adequately designed and maintained utility systems appears to be zero. Published in 2005 by John Wiley & Sons, Ltd.     

Effect of ion doping on the dislocation-related photoluminescence in Si+-implanted silicon

The study is concerned with the effect of the additional implantation of Si samples with C⁺, O⁺, B⁺, P⁺, and Ge⁺ impurity ions followed by annealing at 800°C on the behavior of the dislocation photoluminescence line D1, induced in the samples by implantation with Si⁺ ions at a stabilized temperature followed by annealing in an oxidizing Cl-containing atmosphere. It is established that the intensity of the D1 line strongly depends on the type of incorporated atoms and the dose of additional implantation. An increase in the D1 line intensity is observed upon implantation with oxygen and boron; at the same time, in other cases, the D1 luminescence line is found to be quenched. The mechanisms of such behavior, specifically, the role of oxygen and its interaction with implanted impurities are discussed.     

Postgrowth Annealing of CdTe Layers Grown on Si Substrates by Metalorganic Vapor-Phase Epitaxy

Annealing conditions of CdTe layers grown on Si substrates by metalorganic vapor-phase epitaxy were studied. Typically, 3-μm-thick n-type (211) CdTe layers were annealed for 60 s in flowing hydrogen at atmospheric pressure by covering their surfaces with bulk CdTe wafers. At annealing temperatures above 700°C, improvement of crystal quality was confirmed from full-width at half-maximum values of double-crystal rocking-curve measurements and x-ray diffraction measurements. Photoluminescence measurements revealed no deterioration of electrical properties in the annealed n-CdTe layers. Furthermore, annealing at 900°C improved the performance of radiation detectors with structure of p-like CdTe/n-CdTe/n ⁺-Si substrate.     

Synthesis and photoluminescence properties of the 4H-SiC/SiO2 nanowires

4H-SiC/SiO2 nanowires are synthesized and the temperature-dependent photoluminescence （PL） properties of the nanowires are studied. Their structure and chemical composition are studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and Raman spectra. At room temperature, an ul- traviolet PL peak and a green PL band are observed. From the PL spectrum measured in the temperature range from 80 K to 300 K, the free excition emission, donor bound excition emission and their multiple-phonon replicas have been observed in ultraviolet region, and their origins have been identified. Moreover, it has been found that the temperature dependence of the free exciton peak position can be described by standard expression, and the thermal activation en- ergy values extracted from the temperature dependence of the free exciton and bound exciton peak integral intensity are about 40 meV and 181 meV, respectively.     

<Emphasis Type="Italic">Ab Initio</Emphasis> Studies of Hydrogen Defects in CdTe

Using first-principles calculations based on density functional theory, we have investigated the nature of H defects in CdTe. The formation energy calculations indicate that the ground state position of the H inside the CdTe lattice depends on charge state: the lowest energy position for H⁰ and H⁺ is at the bond center site, while H⁻ prefers the tetrahedral interstitial site with Cd nearest neighbors (T_Cd). We find that H in CdTe acts as an amphoteric impurity. In p-type samples, H is in a positive charge state, acting as a donor to neutralize the free holes in the valence band, and in n-type samples H acquires an electron, compensating the donors in the sample.     

制备工艺对ZnO薄膜的结构与光学性质的影响

利用射频磁控反应溅射技术生长出具有高度晶面(0002)取向的ZnO外延薄膜。通过AFM、XRD、吸收光谱和荧光光谱等测试手段,分别研究分析了不同衬底、不同溅射气氛和退火对ZnO薄膜结构及光学性质的影响。研究表明,在200℃低温生长的硅基ZnO薄膜具有几百纳米的氧化锌准六角结构外形;当氧氩比为4:1(质量流量比)时,吸收谱激子峰最佳;退火后,激子峰(363 nm)加强,同时出现了402 nm的本征氧空位紫光发射。     

掺氮ZnSe外延层的光致发光研究

报导了掺氮ＺｎＳｅ外延层的光致发光,研究了与氮受主有关的发光峰随温度和激发强度的变化关系．１０Ｋ下施主－受主对发光峰随激发强度的增加向高能方向移动,且峰强呈现饱和趋势．在１０～３００Ｋ温度范围光致发光谱表明,随着温度增加,由于激子在受主束缚激子态和施主束缚激子态之间转移,施主束缚激子发光峰强度相对受主束缚激子发光峰强度增加     

非掺杂n型氮化镓外延层的光致发光

研究了热处理对非掺杂 n型氮化镓外延层光致发光谱的影响和光谱中各发光带强度与温度之间的关系 .热处理后 ,光谱中的带边峰和黄光峰的强度较热处理前都有明显降低 .黄光峰强度随温度升高的衰减速度要比带边峰慢得多 .由这些实验结果得出结论 :光谱中的带边峰是由自由激子和束缚在一浅施主能级的束缚激子的谱线重合而成 ,这个浅施主能级很有可能是由氮空位产生 ;黄色荧光的机制应为自由电子或施主能级向深受主能级的跃迁 ,并且黄色荧光肯定和氮化镓中的一内部缺陷产生的深受主能级有关 ,该内部缺陷很有可能是镓空位 .     

非掺杂n型氮化镓外延层的光致发光

研究了热处理对非掺杂n型氮化镓外延层光致发光谱的影响和光谱中各发光带强度与温度之间的关系．热处理后,光谱中的带边峰和黄光峰的强度较热处理前都有明显降低．黄光峰强度随温度升高的衰减速度要比带边峰慢得多．由这些实验结果得出结论：光谱中的带边峰是由自由激子和束缚在一浅施主能级的束缚激子的谱线重合而成,这个浅施主能级很有可能是由氮空位产生;黄色荧光的机制应为自由电子或施主能级向深受主能级的跃迁,并且黄色荧光肯定和氮化镓中的一内部缺陷产生的深受主能级有关,该内部缺陷很有可能是镓空位．     

Self-Assembled CdTe Quantum Dots Grown on ZnTe/GaSb

We report the formation of CdTe self-assembled quantum dots (QDs) on high-quality ZnTe epilayers grown on a GaSb substrate by molecular-beam epitaxy. Reflection high-energy electron diffraction and atomic force micro- scopy measurements clearly show the formation of CdTe QDs. Photoluminescence (PL) measurements on ZnTe-capped CdTe QD samples show a wide distribution of dot size. The temperature dependence of PL spectra indicates much stronger exciton localization in QDs than that in quantum wells.     

Electrical and optical properties of lodine doped CdZnTe layers grown by metalorganic vapor phase epitaxy

Electrical and photoluminescence properties of iodine doped CdZnTe (CZT) layers grown by metalorganic vapor phase epitaxy have been studied. Doped layers showed an n-type conductivity from the Zn composition x=0 (CdTe) to 0.07. Above x=0.07, resistivities of doped layers increased steeply up to 10⁶ Ω-cm. Resistivities of doped CZT layers were higher than those of undoped layers above x=0.6. Photoluminescence intensity of doped layers increased compared to undoped layers. Doped CdTe and ZnTe layers showed neutral donor bound exciton emission lines at the exciton related region. Also, these layers showed an increase in emission intensity at the donor acceptor pair recombination bands. Sharp emission lines were observed in doped CZT layers at around 1.49 eV. These emission lines were considered to be originated from GaAs substrates which were optically excited by the intense emission from doped CZT layers.     

Incorporation and Activation of Arsenic Dopant in Single-Crystal CdTe Grown on Si by Molecular Beam Epitaxy

We report the use of molecular beam epitaxy to achieve p-type doping of CdTe grown on Si(211) substrates, by use of an arsenic cracker and post-growth annealing. A high hole density in CdTe is crucial for high efficiency II–VI-based solar cells. We measured the density of As in single-crystal CdTe by secondary ion mass spectroscopy; this showed that high As incorporation is achieved at low growth temperatures. Progressively higher incorporation was observed during low-temperature growth, presumably because of degradation of crystal quality with incorporation of As at such defect sites as dislocations and defect complexes. After As activation annealing under Hg overpressure, hole concentrations were obtained from Hall measurements. The highest doping level was ～2.3 × 10¹⁶ cm^?3, and near-10¹⁶ cm^?3 doping was readily reproduced. The activation efficiency was ～50%, but further optimization of the growth and annealing conditions is likely to improve this value.     

Direct growth of high-quality thick CdTe epilayers on Si (211) substrates by metalorganic vapor phase epitaxy for nuclear radiation detection and imaging

Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 μm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 μm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n⁺-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.