New techniques in SIMS analysis of HgCdTe materials

This paper presents two new approaches used in the secondary ion mass spectrometry (SIMS) for high sensitivity dopant and impurity analysis in HgCdTe materials, namely, the high mass resolution with dynamic transfer and the MCs⁺ technique. It is shown that better detection limits for As and Cu can be obtained at a level of 2–5e14 at/cm³. The MCs⁺ technique has added advantages of better measurement precision, monitoring Cd composition in the same profile, and small device area analysis capability. Advantages and trade-offs of each technique are discussed and compared. An updated detection limit table for all elements measured in HgCdTe materials using SIMS is also presented.     

Resonance lonization spectroscopy for quantitative and sensitive surface and bulk measurements of impurities in II-VI materials

We have applied resonance ionization spectroscopy for the first time on II-VI materials, Cd, Te, CdZnTe, and HgCdTe for the measurement of trace impurities. It is an analytical technique with extremely high sensitivity, selectivity, dynamic range, and quantitation accuracy. The technique provides virtual freedom from matrix effects and minimizes isobaric and other mass interferences, known to be the shortcomings in secondary ion mass spectroscopy and other mass spectroscopic measurements. Quantitative analysis of Cu in bulk CdZnTe boules has shown Cu concentration in the range low 10¹⁴ to low 10¹⁵ cm^-3 with an average copper content in four different boules near 2 x 10¹⁴ cm^-3. High Cu concentration (1-2 x 10¹⁷ cm^-3) measured in some HgCdTe epitaxial layers correlated with lower Hall mobility in the layer, and in one case the intentionally In-doped, n-type HgCdTe layer turned p-type.     

SIMS在碲镉汞红外焦平面探测器工艺中的应用

文章介绍了二次离子质谱仪的结构及其基本工作原理,并通过对典型应用的分析,介绍了二次离子质谱分析技术在高灵敏度碲镉汞红外焦平面探测器材料和器件制备工艺中的作用,特别是在结探监测和微量杂质监控方面所发挥的重要作用。     

Modeling of Copper SIMS Profiles in Thin HgCdTe

Anomalous secondary-ion mass spectroscopy (SIMS) profiles of copper in thin pieces of HgCdTe are explained using the model used for diode formation by ion milling and ion implantation. In this model, the SIMS ion beam injects mercury interstitials into the HgCdTe as it etches the HgCdTe. The interstitials fill metal vacancies and kick copper off the metal lattice sites. The copper interstitials then diffuse either to the surface being etched, where it is removed and detected by the SIMS instrument, or deeper into the HgCdTe, where it annihilates vacancies. Good agreement between model predictions and experimental SIMS profiles are obtained.     

SIMS analysis of nitrided oxides grown on 4H-SiC

This paper shows for the first time, physical evidence of nitrogen incorporation at the oxide-SiC interface as a result of post-oxidation annealing in nitric oxide (NO). Using secondary ion mass spectroscopy (SIMS) analysis, the location and shape of the nitrogen profile is seen to be almost identical to that found in oxide-silicon interfaces. Close examination of oxygen and carbon SIMS profiles and atomic force microscope scans also indicate a sharper interface when annealing is done using NO compared to inert gases such as N₂ or argon, possibly due to the removal of carbon clusters which form at the interface during oxidation. As in the case of silicon, NO annealing shows great promise as a processing step in the production of device quality gate oxides on SiC.     

Characterization of CdTe for HgCdTe surface passivation

The objectives of this work are to study the physical and chemical structure of CdTe films using secondary ion mass spectrometry (SIMS) and atomic force miroscopy (AFM) and to demonstrate the usefulness of these analytical techniques in determining the characteristics of CdTe-passivation films deposited by different techniques on HgCdTe material. Three key aspects of CdTe passivation of HgCdTe are addressed by different analytical tools: a) morphological microstructure of CdTe films examined by atomic force microscopy; b) compositional profile across the interface determined by Matrix (Te)—SIMS technique; c) concentration of various impurities across the CdTe/HgCdTe structure profiled by secondary ion-mass spectrometry.     

Mode of arsenic incorporation in HgCdTe grown by MBE

The results of arsenic incorporation in HgCdTe layers grown by molecular beam epitaxy (MBE) are reported. Obtained results indicate that arsenic was successfully incorporated as acceptors in MBE-HgCdTe layers after a low temperature anneal. Secondary ion mass spectrometry and Hall effect measurements confirm that arsenic is incorporated with an activation yield of up to 100%. This work confirms that arsenic can be used as an effective dopant of MBE-HgCdTe after a low temperature annealing under Hg-saturated conditions.     

Improved determination of matrix compostion of Hg1−xCdxTe by SIMS

Results are presented to show an improved method for composition characterization of HgCdTe heterostructure using secondary ion mass spectroscopy. This method utilizes the molecular ions CsM⁺ rather than M^± ions. The advantage is that the molecular CsM⁺ ion yield, unlike the atomic M^± ions, is quite insensitive to the matrix material from which they are emitted. Composition of multilayer HgCdTe structure can be determined with excellent accuracy and depth resolution. Layer thickness of HgCdTe heterostructure can also be calibrated.     

砷离子注入体材料碲汞的二次离子质谱分析

用二次离子质谱（ＳＩＭＳ）分析了低能注放（１５０ｋｅＶ）砷在体材料碲镉汞中的分布和注入砷原子在碲镉汞中的热扩散情况,砷在碲镉汞中的分布表现出复杂的多元扩散机制。在缺陷密度（ＥＰＤ）比较低的碲镉汞材料中,砷扩散的主体符合恒定扩散系数的有限源扩散模型,呈现出浓度随深度的高斯分布。而在缺陷密度比较大的碲镉汞材料中,砷的分布呈多段指数型分布,表面出更复杂的多机制扩散特性。     

Analysis of V/III incorporation in nonstoichiometric GaAs and InP films using SIMS

Using secondary ion mass spectrometry (SIMS), we have investigated the excess group V content in GaAs and InP films grown by molecular beam epitaxy at low temperature. Using the inherent depth profiling capability of SIMS, we investigated the V/III ratio in films grown at nominally constant temperatures and also in films grown with stepped temperature profiles. Thickness profiles of the V/III ratio show the effects of intentional temperature changes and of an unintentional drift in the actual substrate temperature during growth. The ability to measure as little as 0.1% excess As and about 0.2% excess P indicates excellent measurement resolution. SIMS analysis is also used to identify a narrow growth temperature range over which InP can be grown with appreciable nonstoichiometry yet remain monocrystalline.     

X-ray diffraction characterization of LPE HgCdTe heterojunction photodiode material

High-resolution x-ray diffraction has been used to measure the composition difference between P and N layers in HgCdTe heterojunction photodiode material grown by liquid phase epitaxy. The composition (band gap) difference is a critical parameter in long wavelength photodiodes because it affects dark current and the formation of photocurrent collection barriers. We find that symmetric 333 reflections cannot resolve the small composition differences of interest. However, by making use of the asymmetric 246 reflection, small composition differences (0.03) can be resolved. There is good agreement between rocking curves and secondary ion mass spectroscopy composition depth profiles, both in the value of the composition difference and in the extent of compositional grading in the top layer. High-resolution x-ray diffraction shows promise as a nondestructive, relatively rapid technique for screening as-grown heterojunction material for carrier collection barriers.     

An improved method for Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te surface chemistry characterization

Hg_1−xCd_xTe surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg_1−xCd_xTe surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-μm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg_1−xCd_xTe processing steps including photolithography, bromine etching, and photoresist stripping.     

Mercury interstitial generation in ion implanted mercury cadmium telluride

Junction formation and stability in ion implanted mercury cadmium telluride critically depend on the ability to generate Hg interstitials. The creation of Hg interstitials is found to strongly depend on the preferred lattice position of the element implanted. Elements that substitute onto the cation sublattice create significantly more Hg interstitials than elements that sit interstitially or on the anion sublattice. Recoils from implant damage also contribute to Hg interstitial formation in heavier mass implants (Z ≥ of mass Zn), but appear to have negligible influence on interstitial generation in implants of lighter ions. The combination of implanting ions of large mass and high solubility on the cation sublattice produces strong Hg interstitial sources. Implants with these ions can form deep junctions even in heavily doped substrates. Junction stability is also improved with the stronger interstitial source.     

Accurate Determination of the Matrix Composition Profile of Hg<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te by Secondary Ion Mass Spectrometry

In this report, we present a new secondary ion mass spectrometry (SIMS) analysis technique to provide accurate Cd composition profiles based on the measurement of HgCs⁺ and CdCs⁺ cluster ions. Study of Hg_1–x Cd _x Te samples with different x values shows that x/(1 − x) is linearly proportional to HgCs⁺/CdCs⁺ over the range of x = 0.2 to x = 0.9. This technique allows us to obtain an accurate Cd profile for a multilayer HgCdTe sample with different x values for each layer using a single standard with known x value. (Received 10/15/06; accepted 2/14/07)     

Hall effect characterization of LPE HgCdTe P/n heterojunctions

The field and temperature dependence of the Hall coefficient has been used to simultaneously extract information about the p and n layers in very long wave length infrared P/n HgCdTe heterojunctions. The field dependence allows the effects of high mobility electrons to be separated from those of low mobility holes. The higher the magnetic field, the higher the sensitivity to the parameters of the P layer. For a maximum magnetic field of 8000 gauss, the hole sheet concentration must be at least five times the electron sheet concentration to obtain accurate results for the P layer. This criterion is satisfied for typical liquid phase epitaxy (LPE) heterostructures. The analysis determines the hole sheet resistance (concentration times mobility), rather than the hole concentration or mobility separately. Independent knowledge of the P layer thickness and the relationship between hole concentration and resistivity are needed to convert the Hall measurement results to hole concentrations. Analysis of the field-dependent Hall data is complicated by the finding that at least three electrons of different mobilities are needed to fit the field dependence of the Hall coefficient in n-type LPE HgCdTe layers. These results are consistent with previous conclusions that electrons with different mobilities are needed to model bulk n-HgCdTe, and with a range of mobilities in the graded composition interface between the LPE layer and CdTe substrate. Consistent results are obtained for the concentrations and mobilities of the three types of electrons in the n-HgCdTe layer with and without the P layer present. N and P type carrier concentrations are also consistent with dopant concentrations measured by secondary ion mass spectroscopy.     

Thermal stability of heavily tellurium-doped InP grown by metalorganic molecular beam epitaxy

The thermal stability of tellurium in InP has been examined in samples doped with Te up to an electron concentration of 1.4 × 10²⁰ cm⁻³. Annealing was conducted using rapid thermal annealing for a period of one minute at temperatures over the range 650–800°C. Secondary ion mass spectroscopy analysis showed virtually no change in the Te profile before and after annealing, even at the highest annealing temperatures. High resolution x-ray diffraction and Hall measurements revealed a general decrease in the lattice strain and carrier concentration for annealing temperatures above 650°C. No evidence of strain relief was found in the form of cross-hatching or through the formation of a dislocation network as examined by scanning electron microscopy or transmission electron microscopy (TEM). These results are most likely due to the formation of Te clusters, though such clusters could not be seen by crosssectional TEM.     

MBE growth and characterization of in situ arsenic doped HgCdTe

We report the results of in situ arsenic doping by molecular beam epitaxy using an elemental arsenic source. Single Hg_1−xCd_xTe layers of x ∼0.3 were grown at a lower growth temperature of 175°C to increase the arsenic incorporation into the layers. Layers grown at 175°C have shown typical etch pit densities of 2E6 with achievable densities as low as 7E4cm⁻². Void defect densities can routinely be achieved at levels below 1000 cm⁻². Double crystal x-ray diffraction rocking curves exhibit typical full width at half-maximum values of 23 arcsec indicating high structural quality. Arsenic incorporation into the HgCdTe layers was confirmed using secondary ion mass spectrometry. Isothermal annealing of HgCdTe:As layers at temperatures of either 436 or 300°C results in activation of the arsenic at concentrations ranging from 2E16 to 2E18 cm⁻³. Theoretical fits to variable temperature Hall measurements indicate that layers are not compensated, with near 100% activation after isothermal anneals at 436 or 300°C. Arsenic activation energies and 77K minority carrier lifetime measurements are consistent with published literature values. SIMS analyses of annealed arsenic doping profiles confirm a low arsenic diffusion coefficient.     

Trace copper measurements and electrical effects in LPE HgCdTe

Recent improvements in sputter initiated resonance ionization spectroscopy (SIRIS) have now made it possible to measure copper in HgCdTe films into the low 10¹³ cm⁻³ range. We have used this technique to show that copper is responsible for type conversion in n-type HgCdTe films. Good n-type LPE films were found to have less than 1 x 10¹⁴ cm⁻³ copper, while converted p-type samples were found to have copper concentrations approximately equal to the hole concentrations. Some compensated n-type samples with low mobilities have copper concentrations too low to account for the amount of compensation and the presence of a deep acceptor level is suggested. In order to study diffusion of copper from substrates into LPE layers, a CdTe boule was grown intentionally spiked with copper at approximately 3 x 10¹⁶ cm⁻³. Annealing HgCdTe films at 360°C was found to greatly increase the amount of copper that diffuses out of the substrates and a substrate screening technique was developed based on this phenomenon. SIRIS depth profiles showed much greater copper in HgCdTe films than in the substrates, indicating that copper is preferentially attracted to HgCdTe over Cd(Zn)Te. SIRIS spatial mapping showed that copper is concentrated in substrate tellurium inclusions 5–25 times greater than in the surrounding CdZnTe matrix.     

Photoelectrochemical capacitance-voltage measurements of 4H-SiC

Photoelectrochemical capacitance-voltage (PC-V) measurements have been successfully made on 4H-SiC material. The 0.05 M KOH solution used as a Schottky contact etches the material with an average roughness of about one percent of the total average etch depth at an etch rate of 1 μm/h. This capability allows the carrier concentration profile of thicker layers with different doping levels to be analyzed. The PC-V measurements are compared with secondary ion mass spectroscopy. The results show agreement with nitrogen doping in the top epitaxial layer and in the substrate. The PC-V measurement also sees an unknown species diffusing out of the substrate into the nominally undoped buffer layer.     

InP/In_xGa_(1-x)As异质结构中Zn元素的扩散机制

采用闭管扩散方式实现了Zn元素在晶格匹配InP/In_(0.53)Ga_(0.47)As及晶格失配InP/In_(0.82)Ga_(0.18)AS两种异质结构材料中的P型掺杂,利用二次离子质谱(SIMS)以及扫描电容显微技术(SCM)对Zn在两种材料中的扩散机制进行了研究.SIMS测试表明:Zn元素在晶格失配材料中的扩散速度远大于在晶格匹配材料中的扩散速度,而SCM测试表明:两种材料中的实际PN结深度与SIMS测得的Zn扩散深度之间存在一定的差值,这是由于扩散进入材料中的Zn元素并没有被完全激活,而晶格失配材料中Zn的激活效率相对更低,使得晶格失配材料中Zn元素扩散深度与PN结深度的差值更大.SCM法是一种新颖快捷的半导体结深测试法,对于半导体器件工艺研究具有重要的指导意义.