Cadmium Zinc Telluride (Cd1-xZnxTe or CZT) is a ternary II-VI compound semiconductor material that has been widely used in infrared detector applications for many years. Due to its lattice spacing, CZT is the substrate of choice for stabilizing Mercury Cadmium Telluride (Hg1-xCdxTe or MCT) crystal layer growth where the lattice matching reduces stress during detector growth processes for high performance infrared detectors and focal plane (FPA) arrays used in guidance systems and a wide array of IR applications. The manufacturing of high performance MCT IR detectors requires CZT substrates of high quality for both bulk and surface conditions thus enabling high quality MCT epitaxial layer crystallinity and low defectivity. In this work, we report on results on bulk CZT material grown using the Travelling Heater Method (THM) that are suitable for infrared focal plane array (IRFPA) detector applications. This proven crystal growth process has been used to manufacture CZT substrates meeting industry requirements of IR transmission, tellurium precipitate size, dislocations and of larger single crystal area. We will present results on chemomechanical (CMP) polishing of CZT substrates of square, rectangular and state-of-the-art round geometries utilizing standard production tool sets that are identical to those used to produce epitaxy-ready surface finishes on related IR compound semiconductor materials such as GaSb and InSb. Surface quality will be assessed by various analytical and microscopy techniques to validate the suitability of this material for epitaxial growth.
In this paper we report on the maturation of large diameter GaSb and InSb substrate production and the key aspects of product quality and process control that have enabled a level of standardization to be achieved that is on par with mass produced compound semiconductor materials such as GaAs and InP. The evolution of commercial production processes for the crystal growth, wafering and epitaxy-ready polishing of antimonide substrates will be discussed together with specific reference to the process tool sets and production methodologies that have transformed a niche material in to one that has set new standards for wafer level product quality, conformity and control. Results will be presented on the production of single crystal >/=6” ingots grown by a modified version of the Czochralski (LEC) technique. Crystal defect mapping will demonstrate that industry standard InSb (211) growth processes have been refined to consistently deliver ultralow dislocation density substrates. Statistical process control data will be presented for large format 5” epitaxy ready finishing processes and compared alongside in-house data for GaAs and InP. Various surface analytical tools are used to characterize 5” InSb and GaSb substrates and our method of providing a unique characterization ‘finger print’ with each substrate discussed. We conclude that improvements in InSb and GaSb product quality and consistency have been driven by the industry’s persistent need to improve device performance and yield. Whilst substrate size requirements in antimonide wafer production may have peaked, we will discuss how to moving to the next step in substrate diameters, 6”, is very attainable and within relatively short timescales too.
Cadmium Zinc Telluride (Cd1-xZnxTe or CZT) is a compound semiconductor substrate material that has been used for infrared detector (IR) applications for many years. CZT is a perfect substrate for the epitaxial growth of Mercury Cadmium Telluride (Hg1-xCdxTe or MCT) epitaxial layers and remains the material of choice for many high performance IR detectors and focal plane arrays that are used to detect across wide IR spectral bands. Critical to the fabrication of high performance MCT IR detectors is a high quality starting CZT substrate, this being a key determinant of epitaxial layer crystallinity, defectivity and ultimately device electro-optical performance. In this work we report on a new source of substrates suitable for IR detector applications, grown using the Travelling Heater Method (THM). This proven method of crystal growth has been used to manufacture high quality IR specification CZT substrates where industry requirements for IR transmission, dislocations, tellurium precipitates and copper impurity levels have been met. Results will be presented for the chemo-mechanical (CMP) polishing of CZT substrates using production tool sets that are identical to those that are used to produce epitaxy-ready surface finishes on related IR compound semiconductor materials such as GaSb and InSb. We will also discuss the requirements to scale CZT substrate manufacture and how with a new III-V like approach to both CZT crystal growth and substrate polishing, we can move towards a more standardized product and one that can ultimately deliver a standard round CZT substrate, as is the case for competing IR materials such as GaSb, InSb and InP.
Cadmium Zinc Telluride (CZT) is an important compound semiconductor material upon which Mercury Cadmium
Telluride (MCT) layers are deposited epitaxially to form structures that are used in high performance detectors covering
a wide infrared (IR) spectral band. The epitaxial growth of high quality MCT layers presents many technical challenges
and a critical determinant of material performance is the quality of the underlying bulk CZT substrate. CZT itself is a
difficult material to manufacture where traditional methods of bulk growth are complex and low yielding, which
constrains the supply of commercially available substrates. In this work we report on the epitaxy-ready finishing of
Travelling Heather Method (THM) grown Cd0.96Zn0.04Te substrates. The THM method is well established for the growth
of high quality CZT crystals used in nuclear, X-ray and spectroscopic imaging applications and in this work we
demonstrate the application of this technique to the growth of IR specification CZT substrates with areas of up to 5 cm x
5 cm square. We will discuss the advantages of the THM method over alternative methods of bulk CZT growth where
the high yield and material uniformity advantages of this technique will be demonstrated. Chemo-mechanical polishing
(CMP) of 4 cm x 4 cm CZT substrates reveals that III-V (InSb/GaSb) like levels of epitaxy-ready surface finishing may
be obtained with modified process chemistries. Surface quality assessments will be made by various surface analytical
and microscopy techniques from which the suitability of the material for subsequent assessment of quality by epitaxial
growth will be ascertained.
Gallium Antimonide (GaSb) is an important Group III-V compound semiconductor which is suitable for use in
the manufacture of a wide variety of optoelectronic devices such as infra-red (IR) focal plane detectors. A
significant issue for the commercialisation of these products is the production of epitaxy ready GaSb, which
remains a challenge for the substrate manufacturer, as the stringent demands of the MBE process, requires a
high quality starting wafer. In this work large diameter GaSb crystals were grown by the Czochralski (Cz)
method and wafers prepared for chemo-mechanical polishing (CMP). Innovative epi-ready treatments and
novel post polish cleaning methodologies were applied. The effect of these modified finishing chemistries on
substrate surface quality and the performance of epitaxially grown MBE GaSb IR detector structures were
investigated. Improvements in the lowering of surface defectivity, maintaining of the surface roughness and
optimisation of all flatness parameters is confirmed both pre and post MBE growth. In this paper we also discuss
the influence of bulk GaSb quality on substrate surface performance through the characterisation of epitaxial
structures grown on near zero etch pit density (EPD) crystals. In summary progression and development of
current substrate polishing techniques has been demonstrated to deliver a consistent improved surface on GaSb
wafers with a readily desorbed oxide for epitaxial growth.
In this paper we report on an industry first; the growth and characterization of 6" diameter indium antimonide (InSb) substrates that are suitable for use in the fabrication of MWIR focal plane infrared detectors. Results will be presented on the production of single crystal 6" InSb ingots grown by the Czochralski (Cz) technique. We will also assess the electrical quality of new 6" InSb crystals and present uniformity information on Hall mobility, resistivity and carrier level from which we will infer comparisons on the relative dark current performance of the material grown. High quality, epitaxy-ready type surfaces have been prepared and we will demonstrate how the key surface quality characteristics of roughness (<0.5nm rms), oxide thickness (<100Å) and flatness (<7 μm TTV) have been maintained across production processes that scale 4" to 6" wafer formats. We conclude by presenting our road map for the development of large area InSb substrates and describe how developments in Czochralski crystal growth and surface finishing technology will support industry's requirements to deliver higher performance, large format IR focal place array type devices.
In this paper we report on an industry first; the growth and characterization of 6” diameter indium antimonide (InSb) substrates that are suitable for use in the fabrication of MWIR focal plane infrared detectors. Results will be presented
on the production of single crystal 6” InSb ingots grown by the Czochralski (Cz) technique. We will also assess the
electrical quality of new 6” InSb crystals and present uniformity information on Hall mobility, resistivity and carrier
level from which we will infer comparisons on the relative dark current performance of the material grown. High
quality, epitaxy-ready type surfaces have been prepared and we will demonstrate how the key surface quality characteristics of roughness (< 0.5 nm rms), oxide thickness (<100 Å) and flatness (<7 μm TTV) have been maintained
across production processes that scale 4” to 6” wafer formats. We conclude by presenting our road map for the development of large area InSb substrates and describe how developments in Czochralski crystal growth and surface finishing technology will support industry’s requirements to deliver higher performance, large format IR focal plane array type devices.
In this paper we describe the bulk crystal growth and characterization of low defect mono-crystalline InSb and GaSb
substrates suitable for use in the epitaxial deposition of infrared detector structures. Results will be presented on the
production of single crystal InSb and GaSb ingots grown by both standard and modified forms of the Czochralski (Cz)
technique. Material quality has been assessed by a new method of fully automated defect recognition microscopy
(DRM) that enables crystallographic defect structures (etch pits) to be mapped and presented in real time. X-Ray
Diffraction (XRD) assessments have been used to derive information on the spatial uniformity of bulk quality and this
shows that very high quality crystals have been grown. Consideration has also been given to the requirements for
manufacture of ≥4" diameter ingots that will be necessary to support the fabrication of very large area, Sb-based detector
structures. The scaling challenges associated with InSb and GaSb production will also be discussed.
Mega-pixel FPAs in both MWIR and LWIR spectral bands based on Sb strained layer superlattices and nBn epitaxial
structures grown on GaSb substrates have recently demonstrated impressive performances at high operating
temperatures. An essential component of SLS epitaxial growth initiation is the starting wafer flatness, smoothness and
haze. Large diameter GaSb wafers must be manufactured meeting these stringent demands and current state-of-the-art
GaSb substrate manufacturing is focused on 100mm wafer diameters. Using a newly developed polishing process,
100mm GaSb substrate manufacturing has resulted in consistent starting wafer peak-to-valley flatness well below 5μm
and surface roughness below Rms of 0.2nm. Final substrate and epitaxial wafer Surfscan mapping (<1000/cm2 surface
defects) and surface roughness (Rms~0.2nm) are presented and compared with measurements of the starting substrates.
This paper evaluates the manufacturing and epitaxial growth on 100mm GaSb substrates that have been processed to
achieve an MBE grown InAsSb-based nBn MWIR photodetector structure.
As size requirements and pixel viabilities for infrared focal plane arrays (IRFPAs) continue to
increase, resolution and sensitivity requirements for high performance advanced imaging systems
must meet or surpass stringent demands. Strain layer superlattice (SLS) grown by molecular beam
epitaxy (MBE) on 100mm GaSb has necessitated changes in crystal processing and finishing
parameters. Device layer growth typically requires a thin (2-5 nm) and highly desorbable surface
oxide on very flat substrates for successful MBE. This study compares the ability for rapid pre-epi
desoprtion of three different chemo-mechanical (CMP) finishes on 100mm n:GaSb: CMP-1 with
sequential double side polished (DSP), CMP-2 with sequential DSP, and CMP-2 with
simultaneous double side polished (S-DSP). X-ray photoelectron spectroscopy (XPS) reveals the
improvement from a CMP-1 (Ga-oxide rich) to CMP-2 (Sb-oxide rich) surface. No difference in
surface chemistry was found between the CMP-2 of the sequential vs. simultaneous DSP. Tropel
flatness measurements of the 100mm n:GaSb substrates show that both DSP and SDSP substrate
batches yield excellent (<5μm) wafer warp. However, initial studies have shown a more consistent
wafer flatness with use of the simultaneous-DSP process. MBE growth on the Sb-rich surface was
examined by high resolution XRD and resulted in a 64.7A periodicity and excellent FWHM (~20
arcsec) which verified the GaSb surface finish effectiveness. The resultant surface finish and
flatness may provide a benefit for larger diameter GaSb IRFPA applications.
Engineered substrates such as large diameter (100mm) GaSb wafers need to be ready years in advance of
any major shift in DoD and commercial technology, and typically before much of the rest of the materials
and equipment for fabricating next generation devices. Antimony based III-V semiconductors are of
significant interest for advanced applications in optoelectronics, high speed transistors, microwave devices,
and photovoltaics. GaSb demand is increasing due to its lattice parameter matching of various ternary and
quaternary III-V compounds, as their bandgaps can be engineered to cover a wide spectral range. For these
stealth and spaced based applications, larger format IRFPAs benefit clearly from next generation starting
substrates. In this study, we have manufactured and tested 100mm GaSb substrates. This paper describes
the characterization process that provides the best possible GaSb material for advanced IRFPA and SLS epi
growth. The analysis of substrate by AFM surface roughness, particles, haze, GaSb oxide character and
desorption using XPS, flatness measurements, and SLS based epitaxy quality are shown. By implementing
subtle changes in our substrate processing, we show that a Sb-oxide rich surface is routinely provided for
rapid desorption. Post-MBE CBIRD structures on the 100mm ULD GaSb were examined and reveals a
high intensity, 6.6nm periodicity, low (15.48 arcsec) FWHM peak distribution that suggests low surface
strain and excellent lattice matching. The Ra for GaSb is a consistent ~0.2-4nm, with average batch wafer
warp of ~4 μm to provide a clean, flat GaSb template critical for next generation epi growth.
A key component for third generation FPA development is the megapixel strain layer superlattice (SLS) structures on
GaSb substrates for advanced infrared detectors. A significant aspect that inhibits widespread application of large format
device growth on GaSb is the starting substrate size. Recently, the Czochralski method resulted in the world's first
100mm GaSb boules. The 100mm GaSb substrates can be ultra-low doped (n~4-9x1015/cm3) for extended IR wavelength
transparency. A plethora of changes to the manufacturing process is required for consistent 100mm GaSb growth and
substrate polishing. In this study, we examined the surface quality of the 100mm GaSb as a function of a standard and
experimental Polish "A" which incorporated an additional CMP step as well as a longer final polish time. Atomic force
microscopy (AFM) and power spectral density (PSD) as a function of polish process measured the surface morphology.
Interferometry was used to analyze free standing wafer flatness. Electron spectroscopy for chemical analysis (ESCA)
determined surface oxide thickness, and successful MBE growth of a 400 period Complimentary Barrier Infrared
Detector (CBIRD) structure assessed SLS based device suitability. The epi structure was examined by x-ray diffraction
(XRD). The low 0.3-0.4nm Ra starting 100mm GaSb roughness values, the wafer flatness ~2.3μm per 16 wafer batch,
the low FWHM SLo = 15.48 arsec of the successful CBIRD epi growth and related high intensity XRD ~6.6nm
periodicity peaks suggest that the modified polish provides the 100mm GaSb with a desirable epi ready character and
excellent surface crystallinity for advanced IRFPA applications.
As size requirements and pixel viabilities for mid-wavelength infrared (MWIR) focal plane arrays (FPAs)
continue to increase, larger InSb substrate diameters are of significant interest. To improve resolution and
sensitivity requirements for high performance infrared focal plane array (IRFPA) imaging systems in the 1-5.4 μm
region (77°K), the surface of new larger diameter (150mm) InSb substrates must meet or surpass stringent
demands. The scale-up to 150mm InSb has necessitated changes in processing and finishing parameters, as device
layer growth by epitaxy or other means often requires a surface roughness average (Ra)~0.1 nm. This study
compares two different CMP finishes for new 150mm diameter InSb wafers and examines their suitability for
IRFPA applications. InSb surface quality was examined by molecular beam epitaxy (MBE), reflective high energy
electron diffraction (RHEED), atomic force microscopy (AFM), thermal X-ray spectroscopy (TXPS), and
multiple crystal x-ray diffraction (XRD) for two distinct CMP finishes on InSb(111)B substrates from the same
crystal boule. This study has shown that differences result in thermal desorption properties with the CMP process,
consistent with differences in surface smoothness and oxide composition. Better surface composition and
crystallinity were produced with a modified CMP process for the 150mm diameter crystals. We conclude that the
differential ratio between the pH, the oxidizer percentage, and the buffering percentage of the modified CMP
process is a benefit to larger diameter InSb IRFPA applications.
GaSb substrates have advantages that make them attractive for implementation of a wide range of infrared (IR) detectors
with higher operating temperatures for stealth and space based applications. A significant aspect that would enable
widespread commercial application of GaSb wafers for very long wavelength IR (VLWIR) applications is the capability
for transmissivity beyond 15 μm. Due largely to the GaSb (antisite) defect and other point defects in undoped GaSb
substrates, intrinsic GaSb is still slightly p-type and strongly absorbs in the VLWIR. This requires backside thinning of
the GaSb substrate for IR transmissivity. An extremely low n-type GaSb substrate is preferred to eliminate thinning and
provide a substrate solution for backside illuminated VLWIR devices. By providing a more homogeneous radial
distribution of the melt solute to suppress GaSb formation and controlling the cooling rate, ultra low doped n:GaSb has
been achieved. This study examines the surface properties and IR transmission spectra of ultra low doped GaSb
substrates at both room and low temperatures. Atomic force microscopy (AFM), homoepitaxy by MBE, and infrared
Fourier transform (FTIR) analysis was implemented to examine material quality. As compared with standard low doped
GaSb, the ultra low doped substrates show over 50% transmission and consistent wavelength transparency past 23 μm
with improved %T at low temperature. Homoepitaxy and AFM results indicate the ultra low doped GaSb has a low
thermal desorbtion character and qualified morphology. In summary, improvements in room temperature IR transmission
and extended wavelength characteristics have been shown consistently for ultra low doped n:GaSb substrates.
The effects of two intrinsic deep levels on electrical compensation and effects of dislocations on carrier mobility in semi-insulating CdTe and CdZnTe radiation detector crystals are reported here. These levels were found in samples grown by conventional Bridgman and high-pressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of Es1 equals 0.27 +/- 0.07 eV, Ed1 equals Ev + 0.735 +/- 0.005 eV and Ed2 equals Ev + 0.743 +/- 0.005 eV. The shallow level (Es1) is associated with dislocations. The first deep level (Ed1) is associated with the doubly ionized Cd vacancy acceptor and the second deep level (Ed2) is associated with the Te-antisite (TeCd). The second deep level (TeCd) was found to electrically compensate the material to produce high resistivity CdTe and CdZnTe, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing. The dislocations were found to affect the mobility of the carriers in the CdTe and CdZnTe crystals.
An irreversible deterioration of CdTe and CdZnTe detectors after heat treatments in the temperature range of 150 - 200 degrees Celsius was reported by several authors; however, the nature of the processes responsible for the detector degradation and increased dark currents is not fully understood. In this study we have prepared CdTe and CdZnTe detectors equipped with Au contacts. The detectors were tested before and after thermal annealing under vacuum. Using combined measurements of current voltage characteristics, low temperature photoluminescence and nuclear spectroscopic measurements, we have attempted to differentiate between the various possible contributions to the detector degradation and elucidate the defect formation process involved.
Thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) were used to study the deep trapping levels in undoped and Sn doped Cd1-xZnxTe crystals. Temperature maximum, varying heating rate and initial rise methods were used to extract activation energies and trapping cross sections of the deep trapping levels in the samples. The concentration of deep levels was estimated from the resistivity data using Neumark model. The pure sample had deep trapping levels with ionization energies of EV + 0.73 eV and EV + 0.74 eV. The concentration of the deep levels was estimated at 500 ppb. These deep levels were associated with intrinsic defects due to Cd vacancies and Te antisites. In the Sn doped samples deep levels at EV + 0.34 eV, EV + 0.55 eV and EV + 0.73 eV were observed. The level at EV + 0.55 eV was associated with Sn, while the EV + 0.34 eV and EV + 0.73 eV levels were associated with Cd vacancies. The concentration of these levels was estimated a 10000 ppb. The pulse height measurements on these samples indicated that Sn doping did not improve the detector performance.
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