Jifeng Wang

Activation of Nitrogen Acceptor in ZnSe Homo‐Epilayer Grown by MOCVD

A ZnSe epitaxial layer doped with nitrogen (N) has been grown on ZnSe substrate at optimized conditions by a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas and ammonia as a dopant source. In order to enhance the activation ratio of nitrogen in ZnSe epitaxial layer, ZnSe:N/ZnSe was annealed in atmosphere with zinc-saturated vapor pressure. Photoluminescence spectra measured at 4.2K from annealed ZnSe : N epilayers showed the stronger I 1 N emission. C-V measurement showed that the ZnSe:N epitaxial layer is of p-type conductivity and the highest net acceptor concentration reaches as high as 6.7 x 10 1 7 cm - 3 . This is the highest value for the ZnSe homosystem epitaxial layers grown by the MOCVD method at present. Furthermore, the ionization energy of nitrogen in ZnSe was estimated to be 109 meV by examining the dependence of DAP emission on the measuring temperature.

Growth and annealing effect of high-quality ZnSe:N/ZnSe by MOCVD

The ZnSe : N epitaxial layers were grown on (1 1 0) ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas, and using ammonia as a dopant source. In order to obtain highly doped ZnSe : N epitaxial layers, the optimum growth and doping conditions were determined by studying the photoluminescence (PL) spectra from the ZnSe epitaxial layers grown at different ammonia flux and VI/II flux ratio. Furthermore, in order to enhance the concentration of active nitrogen in ZnSe epitaxial layer, a rapid thermal anneal technique was used for post-heat-treating. The results show that the annealing temperature of over 1023 K is necessary. Beside, a novel treatment method to obtain a smooth substrate surface for growing high quality ZnSe epitaxial layers is also described.

Wide-Bandgap II-VI Semiconductors: Growth and Properties

Wide-bandgap II–VI compounds are been applied to optoelectronic devices, especially light-emitting devices in the short-wavelength region of visible light, because of their direct gap and suitable bandgap energies. Many methods have been extensively applied to grow high-quality films and bulk single crystals from the vapor and liquid phases.

Annealing Effect of ZnSe:N/ZnSe Grown by Metalorganic Chemical Vapor Deposition

High-quality nitrogen-doped ZnSe (ZnSe:N) epitaxial layers were grown on ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system under optimum growth conditions using hydrogen as a carrier gas and ammonia as a dopant source. In order to enhance the activation efficiency of nitrogen in ZnSe epitaxial layers, ZnSe:N/ZnSe was annealed in melted zinc using the rapid thermal annealing technique. The dependence of net acceptor concentration on annealing duration, VI/II flux ratio, and ammonia flux was examined. The highest net acceptor concentration value calculated from capacitance–voltage (C–V) measurements reached 2×1017 cm-3. This is the highest value obtained so far for ZnSe homosystem epitaxial layers grown by the MOCVD method.

Growth and Evaluation of Cd1-yZnyTe Epilayers on (100) GaAs Substrates by Hot Wall Epitaxy

Growth of Cd1-yZnyTe (CZT) on (100) GaAs by hot wall epitaxy (HWE) is reported for the first time. Nearly stoichiometric CZT epilayers of high quality with y≈0.045 are grown using y=0.2 source and Cd reservoir. Under determined optimum growth conditions, the relationship between quality and thickness of CZT epilayers is compared with that of CdTe epilayers. In the case of layers thicker than 4 µm, a broader full width at half maximum (FWHM) is observed for CZT epilayers than for CdTe epilayers. Also, the deep-level emission in 4.2 K photoluminescence does remains up to 12.8 µm. These results are thought to be due to structural defects that originate from phase separation. The X-ray FWHM values of 150–180 arcsec are obtained at thicknesses greater than 12 µm and are comparable to those obtained by another epitaxial method.

Effect of CdTe Layer Thickness on Hg1-xCdxTe Epilayer Growth by Isothermal Vapor Phase Epitaxy Using (100) CdTe/GaAs Substrates and Void Formation at the Interface

The growth of Hg1-xCdxTe (MCT) epilayers by isothermal vapor phase epitaxy (ISOVPE) on (100) CdTe/GaAs substrates, with various thicknesses of CdTe layers, is reported. Subsequent to ISOVPE growth of MCT layer, we observed the formation of triangular voids at the interface between CdTe and GaAs, which strongly depends on the thickness of the CdTe substrate layers. The problem of void formation and Ga outdiffusion can be overcome by using a sufficiently thick CdTe layer. Although the crystallinity of CdTe layers grown on GaAs by hot wall epitaxy (HWE) is strongly related to thickness, the quality of MCT epilayers is better than that of the substrate layer and is independent of the quality of CdTe layers. This result is attributed to the growth mechanism of ISOVPE being different from other epitaxial methods.

II-IV Semiconductors for Optoelectronics: CdS, CdSe, CdTe

Owing to their suitable band gaps and high absorption coefficients, Cd-based compounds such as CdTe and CdS are the most promising photovoltaic materials available for low-cost high-efficiency solar cells. Additionally, because of their large atomic number, Cd-based compounds such as CdTe and CdZnTe, have been applied to radiation detectors. For these reasons, preparation techniques for these materials in the polycrystalline films and bulk single crystals demanded by these devices have advanced significantly in recent decades, and practical applications have been realized in optoelectronic devices. This chapter mainly describes the application of these materials in solar cells and radiation detectors and introduces recent progress.

Compensation of nitrogen acceptor in ZnSe : N/ZnSe grown by MOCVD

It has been proved that the net acceptor concentration as high as 2 × 10 17 cm - 3 Can be obtained when ZnSe:N homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) are annealed in molten zinc. However, it was found that the emission intensity of two sets of donor-acceptor pair (DAP), shallow DAP (D s AP) and deep DAP (D d AP), increased with the increase in annealing temperature. In order to clarify the compensation effect, the study on the origin and the concentration of deep donor related to the D d AP emission should be carried out. The ionization energies of acceptor and deep donor were estimated to be 101 ± 1 and 44 ± 5 meV, respectively, by examining the dependence of their emission peaks with the excitation intensity. Furthermore, the upper limit of the deep donor concentration was estimated to be 1 × 10 16 cm -3 . This is one magnitude smaller than the net acceptor concentration obtained from the annealed ZnSe: N homoepitaxial layer in our previous paper. By studying on the behaviors of deep donor, the main reason to lead to a low net acceptor concentration in the ZnSe:N epitaxial layers is not attributed to deep donor complex containing nitrogen atoms, but hydrogen passivation, although a part of doped-nitrogen atoms combine with other defects to form complicated defects.

Growth Kinetics of Hg1-xCdxTe Epilayers by Semiclosed Open-Tube Isothermal Vapor Phase Epitaxy

Hg1-xCdxTe (MCT) epilayers were grown on (110) CdTe substrates by semiclosed open-tube isothermal vapor phase epitaxy (ISOVPE). The effects of growth parameters on the surface compositions (x) and thicknesses of the epilayers have been clarified and discussed using the general ISOVPE model. It is found that the growth is limited by the interdiffusion process. The growth kinetics in the interdiffusion control regime was also studied. The growth rate followed the t1/2 dependence expected for interdiffusion-limited processes, and the growth rate constant (A) is given by the Arrhenius form with A0=6.63×10-2 cm2/s and the activation energy Q=1.33 eV. The interdiffusion coefficients for the composition (0.05<x<0.5) and temperature (698 K≤T≤ 773 K) are expressed as D(cm2/s)=550exp (-9.27 x)exp (-1.95 eV/κT) by Fleming-Stevenson analysis.

Effect of Cd Reservoir on the Composition (y) of Cd1-yZnyTe Epilayers Grown by Hot Wall Epitaxy

The relation between the composition (y) and the growth condition is examined for hot wall epitaxy (HWE) of (100) Cd1-yZnyTe (CZT) using a CZT (y=0.2) source and Cd reservoir. A good linear relation between the compositions and their partial pressure ratio is obtained. The composition can be effectively controlled by varying both the source composition and vapor pressure of the Cd reservoir.