Direct growth of CdTe on (100), (211), and (111) Si by metalorganic chemical vapor deposition

CdTe epilayers were grown directly on (100), (211), and (111) silicon substrates by metalorganic chemical vapor deposition (MOCVD). The crystallinity and the growth orientation of the CdTe film were dependent on the surface treatment of the Si substrate. The surface treatment consisted of exposure of the Si surface to diethyltelluride (DETe) at temperatures over 600°C prior to CdTe growth. Direct growth of CdTe on (100) Si produced polycrystalline films whereas (lll)B single crystals grew when Si was exposed to DETe prior to CdTe growth. On (211) Si, single crystal films with (133)A orientation was obtained when grown directly; but produced films with (211)A orientation when the Si surface was exposed to DETe. On the other hand, only (lll)A CdTe films were possible on (111) Si, both with and without Te source exposure, although twinning was increased after exposure. The results indicate that the exposure to a Te-source changes the initial growth stage significantly, except for the growth on (111) Si. We propose a model in which a Te atom replaces a Si atom that is bound to two Si atoms.     

In-situ spectroscopic study of the As and Te on the Si (112) surface for high-quality epitaxial layers

A detailed analysis of the As-exposed Si (112) and subsequent Te exposure was performed. X-ray photoelectron spectroscopy shows that the Te- and As-exposed Si (112) surface had 70% As and 27% Te coverage, respectively. Direct surface coverage measurement with ion scattering spectroscopy (ISS) shows that the Si (111) surface is completely covered by As, and that of the Si (112) had about 78% and 20% coverage of As and Te, respectively. Finally, using ISS shadowing effects, it was found that the Te atoms were positioned mainly on the step edges.     

The structure of the Si (211) surface

Silicon (211) has been proposed as an alternative substrate for CdTe/HgCdTe molecular beam epitaxial growth. Silicon has a clear advantage over other substrates because of its low cost, high strength, and thermal-expansion coefficient, which matches that of the silicon readout integrated circuit. The (211) orientation has been shown to yield high-quality CdTe and HgCdTe/CdTe layers over other orientations. The reconstruction and faceting of the Si (211) surface is poorly understood despite the importance of the (211) orientation. The results of low-energy electron diffraction (LEED) studies have been contradictory, and their conclusions are inconsistent with recent scanning tunneling microscopy (STM) studies. LEED and STM images were used to determine the most probable Si (211) surface facet structure as a function of annealing temperature. Samples annealed at a high temperature (i.e., >1260°C) allowed the formation of ordered LEED spot patterns as opposed to the typically reported $[\bar 111]$ streaks. The pattern in the $[0\bar 11]$ direction gave a consistent 2× (7.68 Å) reconstruction.     

Surface Structure of Molecular Beam Epitaxy (211)B HgCdTe

The as-grown molecular beam epitaxy (MBE) (211)B HgCdTe surface has variable surface topography, which is primarily dependent on substrate temperature and substrate/epilayer mismatch. Nano-ripple formation and cross-hatch patterning are the predominant structural features observed. Nano-ripples preferentially form parallel to the \( [\bar {1}11] \) and are from 0 Å to 100 Å in height with a wavelength between 0.1 μm and 0.8 μm. Cross-hatch patterns result from slip dislocations in the three {111} planes intersecting the (211) growth surface. The cross-hatch step height is 4 ± 1 Å (limited data set). This indicates that only a bi-layer slip (Hg/Cd + Te) in the {111} slip plane occurs. For the deposition of MBE (211)B HgCdTe/CdTe/Si, the reorientation of multiple nano-ripples coalesced into “packets” forms cross-hatch patterns. The as-grown MBE (211)B CdTe/Si surface is highly variable but displays nano-ripples and no cross-hatch pattern. Three types of defects were observed by atomic force microscopy (AFM): needle, void/hillock, and voids.     

Atomic steps on an ultraflat Si(111) surface upon sublimation

The kinetics of atomic steps on an ultraflat Si(111) surface is studied by in situ ultrahigh-vacuum reflection electron microscopy at temperatures of 1050–1350°C. For the first time it is experimentally shown that the rate of displacement of an atomic step during sublimation nonlinearly depends on the width of the adjacent terrace. It is established that the atomic mechanism of mass-transport processes at the surface at temperatures higher than 1200°C is controlled by nucleation and the diffusion of surface vacancies rather than of adsorbed Si atoms. The studies make it possible to estimate the activation energy of the dissolution of vacancies from the surface into the bulk of Si. The estimated activation energy is (4.3 ± 0.05) eV.     

Nucleation of ZnTe/CdTe epitaxy on high-miller-index Si surfaces

Tellurium-adsorption studies were conducted on {111}-type Si surfaces that are off-cut from the {111} in the range of 0–30° on both nonpassivated-and arsenic-passivated Si surfaces. Relative surface coverages as a function of Te exposure time and Si-surface orientation were obtained with in-situ x-ray photoelectron spectroscopy (XPS). The XPS results indicate that Te coverage on arsenic-passivated Si surfaces increases as the step density of the surface increases. In contrast, Te-adsorption studies conducted on nonpassivated-Si surfaces showed no dependence between Te coverage and the surface-step density. Subsequent ZnTe and CdTe molecular-beam epitaxial growth on these high-Miller-index Si surfaces further validate a step-edge nucleation model. Additionally, it was observed that the CdTe-epilayer orientation did not always reproduce the Si-substrate orientation and was a function of both the initial Si surface and the growth conditions used.     

Growth of (111) HgCdTe on (100) Si by MOVPE using metalorganic tellurium adsorption and annealing

(lll)B CdTe layers free of antiphase domains and twins were directly grown on (100) Si 4°-misoriented toward<011> substrates, using a metalorganic tellurium (Te) adsorption and annealing technique. Direct growth of (lll)B CdTe on (100) Si has three major problems: the etching of Si by Te, antiphase domains, and twinning. Te adsorption at low temperature avoids the etching effect and annealing at a high temperature grows single domain CdTe layers. Te atoms on the Si surface are arranged in two stable positions, depending on annealing temperatures. We evaluated the characteristics of (lll)B CdTe and (lll)B HgCdTe layers. The full width at half maximum (FWHM) of the x-ray double crystal rocking curve (DCRC) showed 146 arc sec at the 8 |im thick CdTe layers. In Hg_1−xCd_xJe (x = 0.22 to 0.24) layers, the FWHMs of the DCRCs were 127 arc sec for a 7 (im thick layer and 119 arc sec for a 17 (im thick layer. The etch pit densities of the HgCdTe were 2.3 x 106 cm2 at 7 ^m and 1.5 x 106 cm-2 at 17 um.     

In-situ control of temperature and alloy composition of Cd1−xZnxTe grown by molecular beam epitaxy

In this work in-situ spectroscopic ellipsometry (SE) has been applied for the simultaneous determination of the growth temperature and alloy composition for the epitaxial Cd_1−xZn_xTe(211)/Si(211) structure. The optical dielectric functions of CdTe and Cd_0.96Zn_0.04Te (CZT) epilayers were studied as a function of temperature both ex-situ and in-situ in the range from 1.6 eV to 4.5 eV. We employed parametric models for the simulation of the optical properties of CZT at and between the critical points (CP) E₀, E₀ + Δ₀, E₁, E₁ + Δ₁, E₂(Σ) and E₂(Σ). Critical point energies and line widths for Cd_1−xZn_xTe were obtained through the fitting process, which included both zero order and higher order derivatives of the SE pseudo dielectric function. The dependence of the different critical points on Zn concentration x is discussed. It has been demonstrated that the energy of the weak E₀ + Δ₀ transition can be used to measure composition, while the E₁ energy can be used as a real-time temperature measure. The model parameters were optimized through the simultaneous analysis of multiple data sets, and the temperature dependent model was developed for in-situ application. Our analysis is estimated to produce uncertainties of only ±0.5°C in measuring the temperature and ±0.5% in measuring the composition of only the zero order dielectric function is being fitted. The effects of a surface overlayer, of reflected beam deflections, and of other experimental problems on the overall accuracy, are discussed as well as ways to improve the in-situ SE data quality.     

红外光电子材料碲镉汞异质外延结构理论研究

文章介绍了基于第一性原理的LAPW方法就Hg空位缺陷对碲镉汞材料的电子结构的影响进行了研究。首先选择Hg0. 5 Cd0. 5 Te体系详细分析了Hg空位引起的弛豫,包含Hg空位缺陷体系的电荷密度、成键电荷密度和态密度,得到了碲镉汞材料形成Hg空位情况下的空位第一近邻阴离子悬挂键重整的形式以及Hg空位所形成的双受主能级。计算发现了Hg空位引起第一近邻Te原子5s态能级向高能端移动的现象。 同时,对实验中通常利用As钝化基底表面来有效地控制外延生长的极性进行了研究。本文也介绍了基于密度泛函理论模拟了单个及多个As原子在Si (211)重构表面上的吸附、置换行为,通过系统地计算各种可能的吸附、置换构型,并进一步分析能量、键长等性质,对As在Si(211)表面的钝化机理进行了初步研究探讨。对Cd、Te在As钝化前后Si (211)表面上的吸附行为也进行了研究分析,为外延生长实验中利用As钝化来保证B 面极性的做法提供了一定的理论依据。     

Molecular beam epitaxial growth of HgCdTe on CdZnTe(311)B

A series of n-type, indium-doped Hg_1−xCd_xTe (x∼0.225) layers were grown on Cd_0.96Zn_0.04Te(311)B substrates by molecular beam epitaxy (MBE). The Cd_0.96Zn_0.04Te(311)B substrates (2 cm × 3 cm) were prepared in this laboratory by the horizontal Bridgman method using double-zone-refined 6N source materials. The Hg_1−xCd_xTe(311)B epitaxial films were examined by optical microscopy, defect etching, and Hall measurements. Preliminary results indicate that the n-type Hg_1−xCd_xTe(311)B and Hg_1−xCd_xTe(211)B films (x ∼ 0.225) grown by MBE have comparable morphological, structural, and electrical quality, with the best 77 K Hall mobility being 112,000 cm²/V·sec at carrier concentration of 1.9×10⁺¹⁵ cm⁻³.     

Comparison of HgTe materials grown in (100), (110), (111), and (211) Orientations

We calculated energies required to remove atoms from various configurations on (111), (110), (100), and (211) HgTe surfaces. The excess pair energies for various species are then calculated and are used in a thermodynamic model to study the growth. All energies are obtained using a Green’s function method. The pair distributions are calculated from these energies in a generalized quasi-chemical approximation. The calculated critical temperatures for surface roughness transition are found to be considerably higher than the usual growth temperature of 185°C, so the growth on these surfaces is expected to be layer-by-layer with formation of two-dimensional islands. However, among the surfaces studied, only the (211) surfaces have an attractive binding energy for Hg, making those surfaces suited for better growth. The critical temperature for growth on (21 l)Hg is slightly higher than that for (211)Te, but we also find that Hg sticking coefficient on (21 l)Hg surface is considerably lower than that on (21 l)Te surface. These calculations are consistent with the observed higher growth rate of the (211)Te surface. Our calculations suggest that there will be fewer grown-in vacancies and Te antisites, at the expense of growth rate and sticking coefficient, for crystals grown on (211)Hg surface. We further calculated the Hg and Te vacancy formation energies as functions of surface orientations and layer depth. The cation vacancy formation energies from completed surface regions (islands) are higher than bulk values near anion terminated surfaces and smaller than bulk values near cation terminated surfaces.     

银纳米线在Si(5512)表面上自组装(英文)

由于银不会在硅表面与硅形成任何硅化物,银线成为在具有排状结构的Si(5512)表面自组装纳米线的候选材料。本研究试用不同的银覆盖度和退火温度制作高纵横比的银纳米线。当覆盖度为0.1单原子层,先后两次退火温度分别为500℃和600～700℃,经缓慢冷却过程,非常规则的银链优先吸附在Si(5512)面的四聚体上,从而形成高纵横比的银纳米线。这一自组装的两步退火过程可理解为:低温退火使吸附的银原子与Si表面结合;高温退火提供银原子扩散到Si表面四聚体位置的能量。     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

Metalorganic Chemical Vapor Deposition of CdTe(133) Epilayers on Si(211) Substrates

Single-crystalline CdTe(133) films have been grown by metalorganic chemical vapor deposition on Si(211) substrates. We studied the effect of various growth parameters on the surface morphology and structural quality of CdTe films. Proper oxide removal from the Si substrate is considered to be the principal factor that influences both the morphology and epitaxial quality of the CdTe films. In order to obtain single-crystalline CdTe(133) films, a two-stage growth method was used, i.e., a low-temperature buffer layer step and a high- temperature growth step. Even when the low-temperature buffer layer shows polycrystalline structure, the overgrown layer shows single-crystalline structure. During the subsequent high-temperature growth, two-dimensional crystallites grow faster than other, randomly distributed crystallites in the buffer layer. This is because the capturing of adatoms by steps occurs more easily due to increased adatom mobility. From the viewpoint of crystallographic orientation, it is assumed that the surface structure of Si(211) consists of (111) terrace and (100) step planes with an interplanar angle of 54.8°. This surface structure may provide many preferable nucleation sites for adatoms compared with nominally flat Si(100) or (111) surfaces. The surface morphology of the resulting films shows macroscopic rectangular-shaped terrace—step structures that are considered to be a (111) terrace with two {112} step planes directed toward 〈110〉.     

Si(113)表面电子结构的研究

采用Si_(16)H_(21)和Si_(31)H_(39)原子集团分别模拟Si(113)和Si(111)表面;通过半经验自洽CNDO法计算了两个体系的电子结构.结果表明,Si(113)具有与Si(111)不同的表面态特征.Si(113)表面台阶和台面原子上电荷重新分布,与悬键相关的表面电子态都强烈地定域在表面Si原子上,尤其局域在悬键方向上,并且具有比(111)面上更高的悬健态密度.理论计算结果能解释以前的光电子谱实验.     

偏转角(211)Si基CdTe复合衬底研究

晶面偏角是提高(211) Si基CdTe复合衬底质量的方法之一。通过对偏转角Si基CdTe复合衬底分子束外延工艺的研究,发现2°和3°偏转角(211)Si基CdTe复合衬底在晶体质量方面优于标准(211)Si基CdTe复合衬底,是未来提高Si基CdTe复合衬底质量的新方向。     

As钝化Si(211)表面上Cd、Te原子行为的第一性原理研究

文章采用基于密度泛函理论的CASTEP计算软件模拟了单个及多个As原子在Si(211)重构表面上的吸附、置换行为,通过系统地计算各种可能的吸附、置换构型,并进一步分 析能量、键长等性质,对As在Si (211)表面的钝化机理进行了初步探讨。同时,对Te、Cd在As钝化前后Si (211)表面上的吸附行为也进行了研究。计算结果表明,单个Cd或Te原子都可以稳定地吸附在清洁表面,但在As钝化表面,难以在钝化区域吸附生长。     

Selective Growth of CdTe on Si(211): First-Principle Calculations

The adsorption of CdTe layers on clean and As-passivated Si(211) substrates has been simulated by first-principle calculations in this study. Based on the simulation results, we theoretically show the important roles of the As₄ passivation during the epitaxial growth. Arsenic can saturate part of the dangling bonds and weaken the surface states. The partial passivation finally induces the B-face polarity selection automatically. This conclusion can provide further explanations for the successful growth of large area high-quality CdTe(211)B layers on the Si(211) substrates.     

Influence of arsenic on the atomic structure of the Si(112) surface

The surface science techniques of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) have been used to characterize the clean Si(112) surface and the influence of an As monolayer on the properties and structure of the surface. In agreement with previous studies, the clean surface is found by both LEEd and atomically resolved STM images to be unstable with respect to faceting into other stable planes. Procedures for in-situ deposition of As onto clean Si surfaces were devised and XPS results show that approximately one monolayer of As can be deposited free of any contamination. The As/Si(112) surface is characterized by a sharper LEED pattern than for the clean surface and by STM images characterized by long rows along the direction with a regular width of 1.9 nm. This is consistent with a doubling of the periodicity in the direction of the bulk-terminated unit cell. This implies that As yields a stable but reconstructed Si(112) surface.     

CdZnTe heteroepitaxy on 3″ (112) Si: Interface, surface, and layer characteristics

Tellurium adsorption studies were made on clean and arsenic passivated (112) silicon surfaces. Quantitative surface coverage values for tellurium were determined by Auger electron spectroscopy. Saturation coverage of up to 1.2 monolayers of tellurium could be obtained on a clean (112) silicon surface. On an arsenic passivated (112) Si surface however, the tellurium saturation coverage was limited to only ∼0.3 monolayer. Analysis of the adsorption behavior suggested that tellurium and arsenic chemisorption occurs preferentially at step edges and on terraces, respectively. The study revealed that arsenic passivation led to a significant decrease in the sticking coefficient of tellurium and an increase in it’s surface mobility. A model describing zinc telluride nucleation on a (112) Si surface is proposed. Thin templates of ZnTe followed by Cd_1−xZn_xTe layers were deposited on (112) Si by molecular beam epitaxy (MBE). The characteristics of the MBE Cd_1−xZn_xTe layers were found to be sensitive to the initial ZnTe nucleation and Si surface preparation.