Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 10⁶ cm⁻² for 8 μm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 10⁶ cm⁻². These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64 × 10⁴ Ωm₂ (0 FOV) for devices with 7.8 μm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.     

Te-rich liquid phase epitaxial growth of HgCdTe on Si-based substrates

The growth of high quality (111)B oriented HgCdTe layers on CdZnTe/GaAs/Si and CdTe/Si substrates by Te-rich slider liquid phase epitaxy (LPE) is reported. Although the (111) orientation is susceptible to twinning, a reproducible process yielding twin-free layers with excellent surface morphology has been developed. The electrical properties and dislocation density in films grown on these substrates are comparable to those measured in HgCdTe layers grown on bulk CdTe substrates using the same LPE process. This is surprising in view of the large lattice mismatch that exists in these systems. We will report details of both the substrate and HgCdTe growth processes that are important to obtaining these results.     

CdTe/GaAs(111)B薄膜的MBE生长研究

采用分子束外延方法,在GaAs(111)B衬底上,生长CdTe薄膜,以求研制出用于液相外延生长碲镉汞(HgCdTe)薄膜的CdTe/GaAs(111)B复合衬底.通过理论分析和实验探索,优化了生长温度和Te/Cd束流比等重要生长参数,获得了质量较好的CdTe薄膜,再通过循环热处理,使CdTe/GaAs(111)B复合衬底的质量得到进一步的提高,X-射线回摆曲线半峰宽(FWHM)有明显的降低.为LPE-HgCdTe薄膜的生长打下了较好基础.     

Optical and microstructural characterization of the effects of rapid thermal annealing of CdTe thin films grown on Si (100) substrates

The effects of rapid thermal annealing (RTA) on CdTe/Si (100) heterostructures have been studied in order to improve the structural quality of CdTe epilayers. Samples of CdTe (111) polycrystalline thin films grown by vapor phase epitaxy (VPE) on Si (100) substrates have been investigated. The strained structures were rapidly thermally annealed at 400°C, 450°C, 500°C, 550°C, and 600°C for 10 sec. The microstructural properties of the CdTe films were characterized by carrying out scanning electron microscopy (SEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). We have shown that the structural quality of the CdTe epilayers improves significantly with increasing annealing temperature. The optimum annealing temperature resulting in the highest film quality has been found to be 500°C. Additionally, we have shown that the surface nucleation characterized by the island size distribution can be correlated with the crystalline quality of the film.     

采用(211)碲锌镉衬底制备碲镉汞液相外延薄膜

文中对(211)晶向的CdZnTe衬底进行液相外延生长HgCdTe。获得的碲镉汞液相外延材料的组分为0. 30 ～0. 33,薄膜厚度为10 ～15μm,表面缺陷密度为500cm- 2 ,材料的FWHM达到24弧秒,位错腐蚀坑密度约为2 ×105 cm- 2 ,该材料的表面形貌与采用(111)晶向衬底的HgCdTe外延材料有较大区别。     

The morphology of CdTe deposited by organometallic vapor phase epitaxy: The effect of substrate misorientation

Substrate misorientation and growth temperature influence the morphology of CdTe epilayers grown by organometallic vapor phase homoepitaxy. These effects were investigated by using CdTe{100} and CdTe{100} misoriented by 2, 4, 6, and 8° toward 〈111〉_Te as substrates for growth in the temperature range from 337 to 425°C. Low angle pyramidal facets appeared on films grown on the CdTe(100) surface. The number density of these pyramidal facets decreased to zero as the substrate misorientation angle increased to 4°. At higher misorientation angles, low angle protrusions, resembling fish scales, appeared on the surface. When the temperature was increased, facet size decreased but facet density increased. The film morphology at the high misorientations, however, improved remarkably with increasing temperature. Cross-sectional transmission electron microscopy provided evidence that both the faceted CdTe films and films with a mirror-like finish were epitaxial single crystals with no planar defects. Schwoebel barriers are suggested as the reason for the faceting of the surface grown on CdTe{100}.     

Si/CdTe复合衬底HgCdTe液相外延材料的生长与性能分析

通过改进推舟液相外延技术,成功地在(211)晶向Si/CdTe复合衬底上进行了HgCdTe液相外延生长,获得了表面光亮的HgCdTe外延薄膜.测试结果表明,(211)Si/CdTe复合衬底液相外延HgCdTe材料组分及厚度的均匀性与常规(111)CdZnTe衬底HgCdTe外延材料相当;位错腐蚀坑平均密度为(5～8)×105 cm-2,比相同衬底上分子束外延材料的平均位错密度要低一个数量级;晶体的双晶半峰宽达到70″左右.研究结果表明,在发展需要低位错密度的大面积长波HgCdTe外延材料制备技术方面,Si/CdTe复合衬底HgCdTe液相外延技术可发挥重要的作用.     

In Situ Monitoring of Thermally Induced Resistivity Changes in Silver Thin Films

This study investigates the evolution of electrical and microstructural properties during thermal treatments of silver thin films deposited by wet chemistry on glass substrates. It aims to provide a link between the observed morphological and crystallographic changes based on in situ monitoring of the electrical resistivity evolution. The resistivity of the silver thin films was found to decrease after thermal treatments performed between 110°C and 250°C. This decrease is explained by the elimination of crystallographic defects, highlighted by the decrease of the full-width at half-maximum (FWHM) of the x-ray diffraction peaks, and by the absence of morphological changes for the same temperature interval. After thermal treatments above 250°C, further changes in FWHM and in resistivity are negligible. However, a change in microstructure appears through the formation of voids. Such void formation combined with the increase in the {111}/{200} ratio in samples annealed at temperatures above 250°C is explained by the onset of agglomera- tion.     

X-ray photoelectron spectroscopy study of oxide and Te overlayers on as-grown and etched HgCdTe

X-ray photoelectron spectroscopy has been used to study the low-temperature (<80°C) preparation of HgCdTe surfaces with atomic hydrogen for the purpose of CdTe passivation. Atomic hydrogen was used to etch overlayers and surfaces of liquid phase epitaxy and molecular beam epitaxy HgCdTe, CdTe, HgTe, and Te. Oxide layers were easily removed, while carbon overlayers were resistant to atomic hydrogen etching at low temperature. Both Te and HgTe are etched by atomic hydrogen, with the HgTe etch rate about twice that of Te, while CdTe and ZnTe are not etched. Chemi-mechanical polishing of liquid phase epitaxy HgCdTe left a 10 to 20Å Te overlayer that could be removed with atomic hydrogen. In all cases, exposure of HgCdTe to atomic hydrogen led to surface composition shifts to higher x-value, with an x-value plateau near x～0.6. All observations could be explained in terms of the formation of a 15Å surface layer of CdTe which blocks further etching.     

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.     

Direct growth of CdZnTe/Si substrates for large-area HgCdTe infrared focal plane arrays

Direct epitaxial growth of high-quality 100lCdZnTe on 3 inch diameter vicinal {100}Si substrates has been achieved using molecular beam epitaxy (MBE); a ZnTe initial layer was used to maintain the {100} Si substrate orientation. The properties of these substrates and associated HgCdTe layers grown by liquid phase epitaxy (LPE) and subsequently processed long wavelength infrared (LWIR) detectors were compared directly with our related efforts using CdZnTe/ GaAs/Si substrates grown by metalorganic chemical vapor deposition (MOCVD). The MBE-grown CdZnTe layers are highly specular and have both excellent thickness and compositional uniformity. The x-ray full-width at half-maximum (FWHM) of the MBE-grown CdZnTe/Si increases with composition, which is a characteristic of CdZnTe grown by vapor phase epitaxy, and is essentially equivalent to our results obtained on CdZnTe/GaAs/Si. As we have previously observed, the x-ray FWHM of LPE-grown HgCdTe decreases, particularly for CdZnTe compositions near the lattice matching condition to HgCdTe; so far the best value we have achieved is 54 arc-s. Using these MBE-grown substrates, we have fabricated the first high-performance LWIR HgCdTe detectors and 256 x 256 arrays using substrates consisting of CdZnTe grown directly on Si without the use of an intermediate GaAs buffer layer. We find first that there is no significant difference between arrays fabricated on either CdZnTe/Si or CdZnTe/GaAs/Si and second that the results on these Si-based substrates are comparable with results on bulk CdZnTe substrates at 78K. Further improvements in detector performance on Si-based substrates require a decrease in the dislocation density.     

Passivation of HgCdTe p-n diode junction by compositionally graded HgCdTe formed by annealing in a Cd/Hg atmosphere

Cadmium telluride (CdTe) is being widely used for passivating the HgCdTe p-n diode junction. Instead of CdTe, we tried a compositionally graded HgCdTe as a passivation layer that was formed by annealing an HgCdTe p-n junction in a Cd/Hg atmosphere. During annealing, Cd diffuses into HgCdTe from the Cd vapor, while Hg diffuses out from HgCdTe, forming compositionally graded HgCdTe at the surface. The Cd mole fraction at the surface was constant regardless of the annealing temperature in the range of 250–350°C. Capacitance versus voltage (C-V) curves for p-type HgCdTe that were passivated with compositionally graded HgCdTe formed by Cd/Hg annealing at 260°C showed a smaller flat-band voltage than the one passivated by thermally deposited CdTe, indicative of the better quality of the passivation. A long-wave infrared (LWIR) HgCdTe p-n junction diode passivated by compositionally graded HgCdTe showed about a one order of magnitude smaller R_dA value than the one passivated by thermally deposited CdTe, confirming the effectiveness of the compositionally graded HgCdTe as a passivant.     

Molecular beam epitaxy of CdTe and Hg1-xCdxTe ON GaAs (100)

Using the molecular beam epitaxial (MBE) technique, CdTe and Hg_1-xCd_xTe have been grown on Cr-doped GaAs (100) sub-strates. A single effusion cell charged with polycrystal-line CdTe is used for the growth of CdTe films. The CdTe films grown at 200 °C with a growth rate of ~ 2 μm/hr show both streaked and “Kikuchi” patterns, indicating single crystalline CdTe films are smoothly grown on the GaAs sub-strates. A sharp emission peak is observed at near band-edge (7865 Å, 1.577 eV) in the photoluminescence spectrum at 77 K. For the growth of Hg_1-xCd_xTe films, separate sources of HgTe, Cd and Te are used. Hg_0.6Cd_0.4Te films are grown at 50 °C with a growth rate of 1.7 μm/hr. The surfaces are mirror-smooth and the interfaces between the films and the substrates are very flat and smooth. As-grown Hg_0.6Cd_0.4Te films are p-type and converted into n-type by annealing in Hg pressure. Carrier concentration and Hall mobility of an annealed Hg_0.6Cd_0.4Te film are 1 × 10¹⁷ cm^?3 and 1000 cm²/V-sec at 77 K, respectively.     

Metalorganic chemical vapor deposition CdTe passivation of HgCdTe

CdTe epilayers are grown by metalorganic chemical vapor deposition (MOCVD) on bulk HgCdTe crystals with x ~ 0.22 grown by the traveling heater method (THM). The THM HgCdTe substrates are (111) oriented and the CdTe is grown on the Te face. The metalorganic sources are DMCd and DETe, and the growth is performed at subatmospheric pressure. Ultraviolet (UV) photon-assisted hydrogen radicals pretreatment plays a dominant role in the electrical properties of the resulting heterostructures. The requirements of a good passivation for HgCdTe photodiodes vis-a-vis the passivation features of CdTe/HgCdTe heterostructures are discussed. The effect of valence band offset and interface charges on the band diagrams of p-isotype CdTe/HgCdTe heterostructures, for typical doping levels of the bulk HgCdTe substrates and the MOCVD grown CdTe, is presented. Electrical properties of the CdTe/HgCdTe passivation are determined by capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor test devices, where the MOCVD CdTe is the insulator. It is found that the HgCdTe surface is strongly inverted and the interface charge density is of the order of 10¹²cm² when the CdTe epilayer is grown without the UV pretreatment. With the in-situ UV photon-assisted hydrogen radicals pretreatment, the HgCdTe surface is accumulated and the interface charge density is -4. 10¹¹ cm^-2.     

Microstructural Characterization of HgCdSe Grown by Molecular Beam Epitaxy on ZnTe/Si(112) and GaSb(112) Substrates

Transmission electron microscopy and small-probe microanalysis have been used to investigate the microstructure of HgCdSe thin films grown by molecular beam epitaxy on ZnTe/Si(112) and GaSb(112) substrates. The quality of the HgCdSe material was dependent on the growth temperature and materials flux, independent of the substrate. Samples grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and increased dislocation densities. Improved preparation of the GaSb buffer layer should be developed for future HgCdSe growth on GaSb(112) substrates.     

Substrate quality impact on the carrier concentration of undoped annealed HgCdTe LPE layers

The impact of Te precipitates and impurities, in CdZnTe or CdTe substrates, on grown liquid phase epitaxy (LPE) HgCdTe layer hole concentrations was studied. The carrier concentrations in capped annealed LPE HgCdTe layers grown on CdZnTe substrates with large densities of Te precipitates are frequently significantly higher than those expected for HgCdTe annealed under Hg-deficient conditions. The carrier concentration in the LPE layer, due to the diffusion of copper ions from contaminated CdTe substrates into the layer, is strongly affected by the polarity of the (111)-oriented substrates. Layers grown on the (111)A face showed very high concentrations of Cu, whereas in those grown on the (111)B face normal carrier concentrations were achieved. These phenomena are discussed on the basis of defects formed either in the epilayer or in the layer-substrate interface.     

Nucleation mechanism for epitaxial growth of aluminum films on sapphire substrates by molecular beam epitaxy

Aluminum (Al) epitaxial films with various thicknesses are grown on sapphire substrates by molecular beam epitaxy (MBE). The nucleation evolution of surface morphology and structural property during the growth of Al epitaxial films on sapphire substrates are investigated in detail. It is found that the 10 nm-thick Al epitaxial films grown on the sapphire substrates show the full-width at half-maximum (FWHM) for Al(111) of 0.35° and the root-mean square (RMS) surface roughness of 2.4 nm. When the thickness increases, the surface initially starts to roughen and then becomes smoother. At the same time, the crystal quality of the Al epitaxial films becomes better thanks to the annihilation of dislocations. As the thickness of Al epitaxial films reaches 800 nm, the FWHM for Al(111) is 0.04° and the RMS surface roughness is 0.14 nm, indicating the high crystal quality and flat surface morphology of Al epitaxial films. The corresponding nucleation mechanism of Al epitaxial films grown on sapphire substrates is hence proposed. This work is of great significance for the fabrication of Al-based devices.     

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.     

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.