Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Structural and electrical properties of brush plated ZnTe films

Zinc telluride thin films were deposited by the brush plating technique at a potential of −0.90 V (SCE) on conducting glass and titanium substrates at different temperatures in the range 30–90 °C. The films were polycrystalline in nature with peaks corresponding to the cubic phase. Direct band gap of 2.30 eV was observed. XPS studiers indicated the formation of ZnTe. Depth profiling studies indicated a uniform distribution of Zn and Te throughout the entire thickness. EDAX measurements were made on the films and it was found that there was a slight excess of Te. The carrier concentration was found to vary from 10¹⁴–10¹⁵ cm⁻³ with increase of substrate temperature. The mobility was found to vary from 5 to 60 cm² V⁻¹ s⁻¹.     

MOCVD生长的CdZnTe／ZnTe多量子附的激子光学特性

用常压ＭＯＣＶＤ方法在ＧａＡｓ（１００）衬底上生长了ＣｄＺｎＴｅ／ＺｎＴｅ多量子阱。在室温下，观测到了ＣｄＺｎＴｅ／ＺｎＴｅ多量子阱的三个谱带发光。根据ＣｄＺｎＴｅ／ＺｎＴｅ多量子阱的吸收光谱和不同激发光强下的发光光谱，分别归结ＣｄＺｎＴｅ／ＺｎＴｅ多量子阱中观测到的三个发光谱带于覆盖层发光、ｎ＝１的重空穴激子发光及杂质发光。     

Some spectral response characteristics of ZnTe thin films

Zinc telluride thin films have been grown at room temperature and higher temperature substrates by thermal evaporation technique in a vacuum of 10^-6 torr. A main peak in the photocurrent is observed at 781 nm (1.58 eV) with two lower amplitude peaks on the lower wavelength side and one on higher wavelength side. The evaluated thermal activation energy is found to correspond well with the main spectral peak. From these studies it can be inferred that temperatures up to 453 K is still in the extrinsic conductivity region of the studied ZnTe thin films. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.     

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

Abstract

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Sb掺杂ZnTe薄膜结构及其光电性能

采用真空蒸发工艺在玻璃衬底上制备了ZnTe和掺Sb-ZnTe多晶薄膜,并在氮气气氛中对薄膜进行热处理.分别利用XRD、SEM、紫外-可见分光光度计、霍尔效应测试仪对薄膜的晶体结构、表面形貌、元素组成以及光学、电学性能进行表征,研究Sb掺杂量和热处理对薄膜性能的影响.结果表明:未掺杂薄膜为沿(111)晶面择优生长的立方相闪锌矿结构,导电类型为P型.Sb掺杂并未改变ZnTe薄膜晶体结构和导电类型,但衍射峰强度降低;Sb含量直接影响着Sb在ZnTe中的存在形式,掺Sb后抑制了薄膜中Te和Zn的结合,使薄膜中Te的含量增加;室温下薄膜的光学透过率和光学带隙取决于掺Sb浓度和退火温度,并且掺Sb后ZnTe薄膜的载流子浓度显著增加,导电能力明显增强.     

Structural and electrical properties of evaporated ZnTe thin films

Zinc telluride thin films of various thicknesses are deposited by vacuum evaporation onto glass substrates at room temperature. The X-ray diffraction technique is used to determine the crystalline structure and grain size of the films, respectively. The structure was found to be cubic with preferential orientation along a (1 1 1) plane and crystallite size of about 50-80 nm. The degree of preferred orientation and crystallite size are increased as the film thickness increases. The current density-voltage (J-V) characteristics showed ohmic conduction in the lower voltage range and space-charge-limited conductivity in the higher voltage range. Capacitance measurements indicated that the films have a relative permittivity, ε_r, of approximately 8.19. Further evidence for this conduction process was provided by linear dependence of V_t on d². Analysis of the results yielded hole concentration , which is correlated with the structural properties.     

Development of ZnSexTe1 − x p-type contacts for high efficiency tandem structures

With a band gap of 1.7 eV CdSe is a near-ideal top cell for a tandem solar cell using CuIn_xGa_1 − xSe₂ (CIGS) as the bottom cell. We and others have demonstrated that CdSe has excellent electronic properties that should result in efficiencies of 18%. The primary obstacle to meeting that objective is low Voc due to not having an effective p-contact. We have made some progress in this regard with ZnSe and ZnTe, but each has limitations that limit Voc below the needed 1+ V. ZnSe is not easily doped p-type, and ZnTe's valence band is not as low as desired. In our recent work we have been combining the two binaries to try to get around these limitations. Films are deposited using conventional co-evaporation to be consistent with manufacturing constraints for solar cells. In one approach we are forming the ternary ZnSe_xTe_1 − x. While giving up a bit of ZnSe's favorable valence band location, we hope to enhance dopability. One of the difficulties that we encountered was maintaining stoichiometry for our targeted Te/Se ratio of 1.0. Such films are typically Zn-rich and not dopable. We found deposition techniques that allow access to stoichiometric films with the desired ratio and have measured modest conductivity. We are also investigating superlattice structures as another way of combining the properties of the binaries. This approach avoids competition between the group VI elements during deposition allowing more control over stoichiometry. However, an added difficulty is posed by the activated N dopant environment in the chamber in that it enhances loss of Te during deposition. The superlattice approach provides means of compensation and is producing stoichiometric films, but conductivity is not yet evident.     

Electrodeposition of polycrystalline ZnTe from simple and citrate-complexed acidic aqueous solutions

The cathodic electrodeposition of polycrystalline zinc telluride from aqueous acidic solutions of zinc sulphate and tellurite, under constant or pulsing voltage, is described. Various compositions of the bath were tested with respect to the Zn/Te precursor ratio, including addition of citrate buffer as a complexation agent. Metallic (Ni and Ti) and semiconductor (CdTe and CdSe) electrodes were used as cathode-substrates. The deposits were characterized by XRD, SEM-EDX and FTIR techniques. The formation of compact barrier layers of zinc blend, stoichiometric ZnTe or mixed (Zn,Cd)Te was attained. Pulse plating in a citrate-free bath and constant potential plating from a citrate bath were seen to have the most beneficial effect on the properties of the electrodeposits, as leading to improved microstructure of the ZnTe films, in terms of crystallinity and stoichiometry. Further, the employment of [1 1 1]-oriented CdTe or CdSe substrates featured an epitaxial growth of polycrystalline ZnTe films as well as an improved Zn/Te atomic ratio compared to the metallic substrates.     

Growth of large-diameter ZnTe single crystals by liquid-encapsulated melt growth methods

The 80-mm-diameter ZnTe single crystals were successfully obtained by the liquid-encapsulated Kyropoulos (LEK) method. Both 〈100〉- and 〈110〉-oriented single crystals were reproducibly grown by using ZnTe seed crystals. Furthermore, 80-mm-diameter, 〈100〉 and 〈110〉 ZnTe single crystals were obtained by the pulling method. The etch pit densities (EPDs) of the grown crystals by the LEK and pulling methods were lower than 10,000 cm⁻². The strain in the grown crystals by the pulling method was lower than that of LEK crystals.