In-Situ monitoring of temperature and alloy composition of Hg1−xCdxTe using FTIR spectroscopic techniques

Optical real-time in-situ sensors play a very important role in the processing of semiconductor devices because of their noncontact remote nature and excellent compatibility with UHV systems. In this work, we report on progress in developing an in-situ temperature sensor for HgCdTe structures grown by molecular beam epitaxy (MBE). Based on the Fourier transform infrared (FTIR) spectrometer, this sensor is capable of continuous real-time monitoring of the surface temperature, thickness and alloy composition of HgCdTe epilayers. The accuracy and sensitivity of this FTIR technique were studied in all temperature ranges of interest. Also compared are two different methods of temperature determination obtained from the normalized spectral radiance. The influence of stray radiation and of sample holder rotation on the measurement accuracy have been studied. Reflectivity spectra for HgCdTe/CdZnTe(211) and HgCdTe/CdTe(211)/Si(211) structures have been analyzed in real time in order to determine the layer thickness and alloy composition for growing layers. Also discussed is a multilayer-structure optical model developed to solve the problem of composition determination at early stages of growth. The application of this model for fitting the transmission spectra is demonstrated.     

Far-infrared detector based on HgTe/HgCdTe superlattices

HgTe/Hg_0.05Cd_0.95Te superlattices (SLs) were grown on (112)B oriented Cd_0.96Zn_0.04 Te substrates using molecular beam epitaxy (MBE). The SLs, consisting of 100 periods of 80-Å-thick HgTe wells alternating with 77-Å-thick Hg_0.05Cd_0.95Te barriers, were designed to operate as detectors in the far-infrared (FIR) region. Infrared absorption spectroscopy, high-resolution transmission electron microscopy (TEM), Hall effect measurements, and x-ray diffraction were used to characterize the superlattice layers. A series of annealing experiments were initiated to quantify the temperature-dependent interdiffusion of the HgTe wells and Hg_0.05Cd_0.95Te barriers and consequently their degradation, which shifts the absorption edges of the SLs to higher energies, since a high-temperature ex situ anneal is normally required in order to produce the p-type material required for a photovoltaic detector. Results from infrared absorption spectroscopy, TEM, and Hall effect measurements for the annealed samples are presented. A FIR SLs single-element photoconductive (PC) device was designed and fabricated. Both material characterization and device testing have established the applicability of the HgTe/Hg_0.05Cd_0.95Te SLs for the FIR region.     

Influence of Hydrogenation on Electrical Conduction in HgCdTe Thin Films on Silicon

HgCdTe is the standard state-of-the-art infrared detector material for space applications. HgCdTe-based infrared photon detector performance can be hindered due to the presence of bulk crystal defects and dangling bonds at surfaces or interfaces. Passivation of such bulk defects and surfaces can potentially improve detector performance by saturating dangling bonds in dislocation cores and at surfaces. Indeed, results showing improvement of HgCdTe current–voltage characteristics after hydrogenation have been reported. Here we use multiple-carrier fitting of Hall-effect data, acquired under variable magnetic field strengths and sample temperatures, to investigate the physical influence of hydrogenation, as a passivation procedure, on HgCdTe crystalline thin films on Si(211) substrates. We find: (1) evidence of multiple active electrical carrier species in all samples, (2) evidence of surface electrical conduction before and after hydrogenation, and (3) changes in carrier concentration and mobility induced by hydrogenation.     

Growth of high quality CdTe on Si substrates by molecular beam epitaxy

We have systematically studied the growth of CdTe (lll)B on Si(001)with different atomic step structures, defined uniquely by miscut tilt angle and direction. X-ray double crystal rocking curve (DCRC) analysis has been used to evaluate the crystalline quality and twin content of the films. High-resolution electron microscopy has been used to examine the CdTe(lll)B/Si(001) interface and to follow the microstructural evolution as a function of distance from the interface. Our results show that the formation of double domains and twins is very sensitive to the tilt parameters. When growth conditions are optimized, twins are not observed at distances greater than about 2.5 microns from the substrate surface. The best quality films exhibit a DCRC FWHM of 60 arc sec, for a film thickness of 17 μm, the lowest value ever reported for heteroepitaxial growth of CdTe on Si or GaAs. In efforts to improve the nucleation process, precursors such as Te and As have been used, and we have shown that they improve the stability of the heterointerface.     

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.     

Two-dimensional multiwavelength fluorescence spectra of dipicolinic acid and calcium dipicolinate

Dipicolinic acid (DPA) and the Ca2+ complex of DPA (CaDPA) are major chemical components of bacterial spores. With fluorescence being considered for the detection and identification of spores, it is important to understand the optical properties of the major components of the spores. We report in some detail on the room-temperature fluorescence excitation and emission spectra of DPA and its calcium ion complex and provide a comparison of the excitation-emission spectrum in a dry, wet paste and aqueous form. DPA solutions have weak, if any, fluorescence, with increased fluorescence when the DPA is dry. After exposure to a broad source UV light of the DPA, wet or dry, we observe a large increase in fluorescence with a maximum intensity emission peak at around 440 nm for excitation light with a wavelength of around 360 nm. There is a slight blueshift in the absorption spectra of UV-exposed DPA from the unexposed DPA solution. CaDPA in solution shows a slight fluorescence with increased fluorescence in the dry form, and a substantial increase of fluorescence was observed after UV exposure with an emission peak of around 410 nm for excitation around 305 nm. The detailed excitation-emission spectra are necessary for better interpretation of the fluorescence spectra of bacterial spores where DPA is a major chemical component.     

Nonequilibrium Operation of Arsenic Diffused Long-Wavelength Infrared HgCdTe Photodiodes

We demonstrated a device with a unique planar architecture using a novel approach for obtaining low arsenic doping concentrations in long-wavelength (LW) HgCdTe on CdZnTe substrates. HgCdTe materials were grown by molecular beam epitaxy (MBE). We fabricated a p-on-n structure that we term P ⁺/π/N ⁺ where the symbol “π” is to indicate a drastically reduced extrinsic p-type carrier concentration (on the order of mid 10¹⁵ cm⁻³); P ⁺ and N ⁺ denote a higher doping density, as well as a higher energy gap, than the photosensitive base π-region. Fabricated devices indicated that Auger suppression is seen in the P ⁺/π/N ⁺ architecture at temperatures above 130 K and we obtained a saturation current on the order of 3 mA on 250-μm-diameter devices at 300 K with Auger suppression. Data shows that about a 50% reduction in dark current is achieved at 300 K due to Auger suppression. The onset of Auger suppression voltage is 450 mV at 300 K and 100 mV at 130 K. Results indicate that a reduction of the series resistance could reduce this further. A principal challenge was to obtain low p-type doping levels in the π-region. This issue was overcome using a novel deep diffusion process, thereby demonstrating successfully low-doped p-type HgCdTe in MBE-grown material. Near-classical spectral responses were obtained at 250 K and at 100 K with cut-off wavelengths of 7.4 μm and 10.4 μm, respectively. At 100 K, the measured non-antireflection-coated quantum efficiency was 0.57 at 0.1 V under backside illumination. Received November 7, 2007; accepted March 19, 2008