J. A. Roth

Laser-Induced Solid Phase Crystallization in Amorphous Silicon Films

We review recent work on the kinetics of laser-induced solid phase epitaxial crystallization of silicon as determined from time-resolved reflectivity measurements. Specific topics which are addressed include: the intrinsic kinetics of solid phase epitaxy (SPE) in ion-implanted and UHV-deposited films; SPE rate enhancement by implanted dopant atoms and the effects of electrical compensation on the SPE rate; and the temperature dependence of SPE and competing processes in samples containing impurity atoms at concentrations exceeding the solid solubility limit. The high temperature kinetics results are compared with predictions from transition state theory and are discussed with respect to a proposed depression in the amorphous Si melting temperature.

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.

Third Generation FPA Development Status at Raytheon Vision Systems

Raytheon Vision Systems (RVS) is developing two-color, large-format infrared FPAs to support the US Armys Third Generation FLIR systems. RVS has produced 640 x 480 two-color FPAs with a 20 micron pixel pitch. Work is also underway to demonstrate a 1280 x 720 two-color FPA in 2005. The FPA architecture has been designed to achieve nearly simultaneous temporal detection of the spectral bands while being producible for pixel dimensions as small as 20 microns. Raytheons approach employs a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). This ROIC is coupled to bias-selectable two-color detector array with a single contact per pixel. The two-color detector arrays are fabricated from MBE-grown HgCdTe triple layer heterojunction (TLHJ) wafers. The single indium bump design is producible for 20 μm unit cells and exploits mature fabrication processes that are in production at RVS for Second Generation FPAs. This combination allows for the high temporal and spatial color registration while providing a low-cost, highly producible and robust manufacturing process. High-quality MWIR/LWIR (M/L) 640 x 480 TDMI FPAs with have been produced and imaged from multiple fabrication lots. These FPAs have LWIR cutoffs ranging to 11 micron at 78K. These 20 micron pixel FPAs have demonstrated excellent sensitivity and pixel operabilities exceeding 99%. NETDs less than 25 mK at f/5 have been demonstrated for both bands operating simultaneously.

Direct molecular-beam epitaxial growth of ZnTe(100) and CdZnTe(100)/ZnTe(100) on Si(100) substrates

Epitaxial structures of ZnTe(100) and CdZnTe(100)/ZnTe(100) have been deposited by molecular‐beam epitaxy onto Si(100) substrates misoriented from 0° to 8° towards the [011] direction. The films were characterized with x‐ray diffraction, photoluminescence spectroscopy, optical microscopy, and stylus profilometry. Single‐crystal CdZnTe(100) films comparable in structural quality to those obtained with growth on GaAs/Si composite substrates have been demonstrated on both 4° and 8° misoriented Si with the use of ZnTe buffer layers. X‐ray rocking curves with FWHM less than 300 arcsec for ZnTe (400) and less than 160 arcsec for CdZnTe(400) have been obtained for as‐grown films. Specular surface morphologies, superior to those obtained on GaAs/Si composite substrates, are also observed.

In situ temperature control of molecular beam epitaxy growth using band-edge thermometry

Band-edge thermometry is becoming an established noncontact method for determining substrate temperature during molecular beam epitaxy. However, with this technique thin-film interference and/or absorption in the growing epilayer can cause shape distortions of the spectrum that may be interpreted erroneously as real temperature shifts of the substrate. An algorithm is presented that uses the width of the spectrum to correct for apparent temperature errors caused by interference and absorption in the epilayer. This correction procedure is tested on substrate temperature data taken during the growth of a λ=930 nm resonant cavity, where the apparent substrate temperature oscillates ±5 °C during the growth of the mirror stacks. These oscillations are reduced to ±3 °C using the correction algorithm. A recently developed model for the substrate temperature dynamics in molecular beam epitaxy shows that roughly ±1 °C of the remaining ±3 °C temperature oscillations are real. Band-edge thermometry is also used to control the substrate temperature to within ±2 °C during the growth of near-lattice-matched InGaAs on InP, whereas the same growth under constant thermocouple temperature would result in a 50 °C rise in the actual substrate temperature.

Closed-loop control of resonant tunneling diode barrier thickness using in situ spectroscopic ellipsometry

We examine the use of spectroscopic ellipsometry (SE) for fully automated, in situ real-time control of the barrier thickness for resonant tunneling diodes (RTDs) grown by molecular-beam epitaxy (MBE). The RTDs in this study utilize AlAs barriers, an InGaAs well with an InAs subwell, and lattice-matched InGaAs spacer layers, grown on an InP(100) substrate. Pseudodielectric functions for the strained AlAs barriers were generated from SE data acquired during MBE growth, using simultaneous photoemission oscillation measurements for in situ thickness calibration. For closed-loop control, the measured dielectric functions were utilized in conjunction with a virtual substrate model to derive the instantaneous layer thickness from real-time SE data. Repeatability was tested by growing several series of AlAs barriers and complete RTDs under fully automated control, with shutter actuation based on the real-time thickness determined from SE. The results show that by using SE for real-time control, barrier thickness...

Status of two-color and large format HgCdTe FPA technology at Raytheon Vision Systems

Raytheon Vision Systems (RVS) is developing two-color and large format single color FPAs fabricated from molecular beam epitaxy (MBE) grown HgCdTe triple layer heterojunction (TLHJ) wafers on CdZnTe substrates and double layer heterojunction (DLHJ) wafers on Si substrates, respectively. MBE material growth development has resulted in scaling TLHJ growth on CdZnTe substrates from 10cm2 to 50cm2, long-wavelength infrared (LWIR) DLHJ growth on 4-inch Si substrates and the first demonstration of mid-wavelength infrared (MWIR) DLHJ growth on 6-inch Si substrates with low defect density (<1000cm-2) and excellent uniformity (composition<0.1%, cut-off wavelength Δcenter-edge<0.1μm). Advanced FPA fabrication techniques such as inductively coupled plasma (ICP) etching are being used to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. Recent two-color detectors with MWIR and LWIR cut-off wavelengths of 5.5μm and 10.5μm, respectively, exhibit significant improvement in 78K LW performance with >70% quantum efficiency, diffusion limited reverse bias dark currents below 300pA and RA products (zero field-of-view, +150mV bias) in excess of 1×103 Ωcm2. Two-color 20μm unit-cell 1280×720 MWIR/LWIR FPAs with pixel response operability approaching 99% have been produced and high quality simultaneous imaging of the spectral bands has been achieved by mating the FPA to a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). Large format mega pixel 20μm unit-cell 2048×2048 and 25μm unit-cell 2560×512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30μm unit-cell 256×256 LWIR FPAs with 10.0-10.7μm 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.

Substrate temperature measurement by absorption-edge spectroscopy during molecular beam epitaxy of narrow-band gap semiconductor films

A novel method is described for utilization of absorption-edge spectroscopy (ABES) to monitor the temperature of a semiconducting substrate during molecular beam epitaxy (MBE) of a film with a band gap narrower than that of the substrate. Conventional ABES cannot be used for substrate temperature determination with narrow-band gap epilayers that are sufficiently thick so as to be opaque in the wavelength range corresponding to the substrate band gap. However, we show that by inserting a reflecting layer (or layers) between the substrate and overlying narrow-band gap epilayer, ABES temperature measurements can be carried out in reflection from the backside of the substrate, even in the presence of arbitrarily thick narrow-band gap epilayers. This approach is demonstrated for MBE growth of InAs on GaAs substrates and also for HgCdTe on CdZnTe substrates, and is shown to be accurate to ±2 °C over temperature spans of 300 and 120 °C, respectively, for these two material systems in the vicinity of the typical ...

Two-color HgCdTe infrared staring focal plane arrays

Raytheon Vision Systems (RVS) in collaboration with HRL Laboratories is contributing to the maturation and manufacturing readiness of third-generation two-color HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256x256 30μm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral regions. FPAs configured for MWIR/MWIR, MWIR/LWIR and LWIR/LWIR detection are used for target identification, signature recognition and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer-heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all two-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single mesa, single indium bump, and sequential mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.

The Effect of Hydrogen on the Kinetics of Solid Phase Epitaxy in Amorphous Silicon

This paper discusses the intrusion of H into a-Si layers during solid phase epitaxy and the effect of this H on the growth kinetics. We show that during annealing in the presence of water vapor, H is continuously generated at the oxidizing a-Si surface and diffuses into the amorphous layer, where it causes a reduction in the epitaxial growth rate. The measured variation of growth rate with the depth of the amorphous/crystal interface is correlated with the concentration of H at the interface. The diffusion coefficient for H in a-Si is determined by comparing measured depth profiles with calculated values. Hydrogen intrusion is observed even in layers annealed in vacuum and in inert gas ambients. Thin (