L. O. Bubulac

University of California, Los Angeles, Los Angeles, CA, United States

Are you L. O. Bubulac?

Claim your profile

Publications (77)135.43 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The crystalline structure and impurity profiles of HgCdTe/CdTe/alternate substrate (AS; Si and GaAs are possibilities) and CdTe/AS were analyzed by secondary-ion mass spectrometry, atomic force microscopy, etch pit density analysis, and scanning transmission electron microscopy. Impurities (Li, Na, and K) were shown to getter in as-grown CdTe/Si epilayers at in situ Te-stabilized thermal anneal (~500°C) interfaces. In HgCdTe/CdTe/Si epilayers, indium accumulation was observed at Te-stabilized thermal anneal interfaces. Impurity accumulation was measured at HgCdTe/CdTe and CdTe/ZnTe interfaces. Processing anneals were found to nearly eliminate the gettering effect at the in situ Te-stabilized thermal anneal interfaces. Impurities were found to redistribute to the front HgCdTe/CdTe/Si surface and p–n junction interfaces during annealing steps. We also investigated altering the in situ Te-stabilized thermal anneal process to enhance the gettering effect.
    Journal of Electronic Materials 11/2013; 42(11). · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A model is proposed to explain disparities found in the operability values and histograms for long-wavelength infrared HgCdTe focal-plane arrays fabricated on Si substrates compared with those fabricated on CdZnTe. The starting point for the model is the close agreement between the aerial density of discrete species (particles, contamination spots, crystalline defects on Si surface) in various interfaces in the HgCdTe/CdTe/Si structure and the density of failed pixels in the array. The density of discrete species is acquired by applying a newly developed variation of the secondary-ion mass spectrometry (SIMS) depth-profiling technique to samples that have been deuterated to enhance detection. A mechanism of selective activation of threading dislocations in a HgCdTe layer on Si is proposed to link discrete species with failed detector pixels.
    Journal of Electronic Materials 01/2011; 40(3):280-288. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Transmission electron microscopy and small-probe microanalysis have been used to investigate the microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures. Thin ZnTe buffer layers and subsequent thick CdTe layers were grown on Si(211) substrates using molecular beam epitaxy. Many {111}-type stacking faults were found to be present throughout the entire ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Local lattice parameter measurement and elemental profiles indicated that some intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd,Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively.
    Journal of Electronic Materials 01/2011; 40:1733-1737. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: High-quality (112)B HgCdTe/Si epitaxial films with a dislocation density of ∼9 × 105 cm−2 as determined by etch pit density (EPD) measurements have been obtained by thermal cyclic annealing (TCA). The reduction of the dislocation density by TCA has led to a simple rate-equation-based model to explain the relationship between dislocation density and TCA parameters (time, temperature, and number of anneals). In this model, dislocation density reduction is based on dislocation coalescence and annihilation, assumed to be caused by dislocation motion under thermal and misfit stress. An activation energy for dislocation motion in n-type (112)B HgCdTe/Si of 0.93 ± 0.1 eV was determined. This model with no adjustable parameters was used to predict recent TCA annealing results.
    Journal of Electronic Materials 01/2011; 40(8):1847-1853. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have performed a detailed study of dark current versus voltage to understand existing limitations in dark current and address the nonuniformity of dark current in devices fabricated on HgCdTe grown on silicon substrates. One interesting observation is that trap-assisted tunneling, g-r currents, are not found close to zero bias in certain devices. Devices from the low end of the R 0 A distribution show heavy shunting paths close to zero bias. We believe that these shunting paths may be the limiting cause of tail distributions in fabricated focal plane array tail distributions. Possible causes for these shunting paths are surface charges associated with dislocation cores and impurity gettering at dislocation cores. The measured non-anti-reflection (AR)-coated quantum efficiency (QE) was 0.576 at 78K and displays the classical response versus wavelength. The measured QE on isolated single devices is consistent with the 256×256 focal-plane array mean QE. Obtained average dark currents are on the order of mid 10−5Acm–2, which is one order of magnitude higher than dark currents obtained from arrays on lattice-matched substrates. On average, arrays on lattice-mismatched substrates show performance characteristics inferior to those of arrays fabricated on lattice-matched substrates. This inferior performance is due to array pixel operability, as can be seen from the tail of the distribution and the average dark currents, which are one order of magnitude higher than those obtained on lattice-matched substrates. KeywordsHgCdTe-Si-curent–voltage-band-to-band tunneling-shunt-trap-assisted tunneling
    Journal of Electronic Materials 01/2010; 39(7):1110-1117. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report an assessment of the reproducibility of the HF cleaning process and As passivation prior to the nucleation of ZnTe on the Si(211) surface using temperature desorption spectroscopy, ion scattering spectroscopy, and electron spectroscopy. Observations suggest full H coverage of the Si(211) surface with mostly monohydride and small amounts of dihydride states, and that F is uniformly distributed across the top layer as a physisorbed species. Variations in major contaminants are observed across the Si surface and at the CdTe-ZnTe/Si interface. Defects act as getters for impurities present on the Si surface, and some are buried under the CdTe/ZnTe heterostructure. Overall, the data show evidence of localized concentration of major impurities around defects, supporting the hypothesis of a physical model explaining the electrical activation of defects in long-wave infrared (LWIR) HgCdTe/CdTe/Si devices. KeywordsCdTe-HgCdTe-Si(211)-tail distribution-molecular-beam epitaxy (MBE)-x-ray photoelectron spectroscopy (XPS)-scanning Auger microscopy (SAM)-ion scattering spectroscopy (ISS)
    Journal of Electronic Materials 01/2010; 39(7):951-957. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Deuterium (as well as other hydrogen isotopes) binds with a wide range of morphological defects in semiconductors and, as such, becomes distributed similarly to those defects. Thus, the deuterium profile within the sample serves as the basis of a technique for defect mapping known as amethyst wafer mapping (AWM). The efficiency of this technique has been demonstrated by evaluation of ion-induced damage in implanted Si, as well as as-grown defects in HgCdTe (MCT) epilayers. The defect tagging or decoration capability of deuterium is largely material independent and applicable to a wide range of defect morphologies. A number of analytical techniques including ion channeling and etch pit density measurements were used to evaluate the AWM results. KeywordsAWM-HgCdTe-hydrogenation-IRFPA-defect tagging
    Journal of Electronic Materials 01/2010; 39(7):930-935. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The electrical performance of HgCdTe/Si photodiodes is shown not to have a direct relationship with the dislocation density as revealed by defect etching. This has led to an equivalent circuit model to explain the relationship of the dislocation density and the electrical test data. A new (112)B HgCdTe/CdTe/Si and CdTe/Si etch pit density (EPD) etch has been demonstrated. The new etch has been used to look for distinctive features which may be responsible for the poor electrical performance of individual diode pixels. The new etch chemistry also reduces the surface roughness of the etched epilayer and makes EPD determination less problematic. The new (to HgCdTe) technique of electrostatic force microscopy has also been used to analyze the electrical properties of dislocations. KeywordsHgCdTe/CdTe/Si-molecular beam epitaxy-atomic force microscopy-electrostatic force microscopy-etch pit density
    Journal of Electronic Materials 01/2010; 39(7):1080-1086. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements all indicate an approximate factor of ten increase in the Everson etch pit density (EPD) over standard Nomarski microscopy Everson EPD determination. A new (112)B CdTe/Si EPD etch has also been demonstrated which reduces the surface roughness of the etched epilayer and makes etch pit density determination less problematic.
    Journal of Electronic Materials 01/2009; 38(8):1771-1775. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Preliminary results of a study of the hydrogenation of HgCdTe epilayers grown by molecular beam epitaxy on Si substrates using a glow-discharge plasma are presented. The aim of the program is to employ H to passivate the detrimental opto-electronic effects of threading dislocations present in the HgCdTe epilayers. Secondary ion mass spectroscopy depth profiling has been performed to characterize 1H and 2H incorporation. It has been found that H can be controllably incorporated in HgCdTe epilayers to levels in the 1014 cm−3 to 1018 cm−3 range while maintaining the sample at temperatures lower than 60°C. Profiles indicate that H accumulates in regions of known high defect density or in highly strained regions. Analysis of the H depth profile data indicates that the current density-time product is a good figure of merit to predict the H levels in the HgCdTe epilayer. There are progressive differences in the 1H and 2H uptake efficiencies as a function of depth. Magneto-Hall measurements show consistently higher mobilities at low temperatures for majority carriers in hydrogenated samples.
    Journal of Electronic Materials 05/2006; 35(6):1465-1469. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present extended results on the use of a hydrogen plasma to passivate the effects of defects in long-wave ir HgCdTe/Si. Annealed and as-grown epilayers, in situ doped with indium, were exposed to a hydrogen plasma generated in an electron cyclotron resonance (ECR) reactor. Secondary ion mass spectrometry was used to measure the extent of hydrogen incorporation into the epilayers. Hall and photoconductive lifetime measurements were used to assess the efficacy of passivation. The passivation of defects responsible for the scattering and recombination of electrical carriers was observed for most ECR conditions over a range of dislocation densities.
    Journal of Electronic Materials 05/2006; 35(6):1385-1390. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the results of using an electron cyclotron resonance (ECR) plasma to incorporate hydrogen into long wavelength infrared HgCdTe layers grown by molecular beam epitaxy. Both as-grown and annealed layers doped in situ with indium were hydrogenated. Secondary ion mass spectroscopy confirmed the incorporation of hydrogen. Hall and photoconductive lifetime measurements were used to assess the effects of the hydrogenation. Increases in the electron mobilities and minority carrier lifetimes were observed for almost all ECR conditions.
    Applied Physics Letters 02/2006; 88:062106. · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The authors have fabricated p-on-n LWIR HgCdTe diodes by extrinsically doping a double-layer heterostructure grown by metalorganic chemical vapour deposition (MOCVD) using the interdiffused multilayer process (IMP) on GaAs substrates. The p-side of the junction was obtained by a fast As diffusion, from an ion-implanted source, ahead of the heterointerface when the annealing was performed at very low partial pressure of Hg. Examples of photodiodes fabricated by this process are given. Thus, for devices processed by As implant/diffusion, an R0A of 24.2 Omega cm2 for a cut-off wavelength of 11.7 mu m at 77 K and a quantum efficiency of 51% were obtained for a mesa unpassivated structure, and an R0A of 91 Omega cm2 for a cut-off wavelength of 10.3 at 77 K and a quantum efficiency of 50% were obtained for a planar structure. At 77 K the transport mechanisms limiting the device performance near zero bias have appeared to be thermal processes (ideality factor n between 1 and 2). However, other mechanisms evident in reverse bias such as shunt and tunnelling, may also operate.
    Semiconductor Science and Technology 12/1998; 8(1S):S270. · 1.92 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low-leakage high-performance photovoltaic detectors were fabricated from long-wavelength infrared (LWIR) HgCdTe epitaxial material grown by metal-organic chemical vapour deposition (MOCVD) on GaAs substrates. Layers were grown by two different MOCVD techniques, a conventional alloy growth and an interdiffused multiple-layer growth. MOCVD HgCdTe layers were characterised by background electron concentrations of (1-3)*1015 cm-3 with electron mobilities up to 200000 cm2 V-1 s-1 at 77 K. Surface and cross-section dislocation densities were 106-107 cm-2 and occasionally (4-5)*105 cm-2. GaAs substrates were (100) and (111B) misoriented towards 110. The layer structure in which the devices were fabricated consisted of an absorbing LWIR HgCdTe layer grown on a buffer of CdTe. A thin layer (1-2 mu m) of wide-band-gap HgCdTe was grown last. Devices were fabricated in a single as well as double layers using a mesa geometry with a native oxide passivation. Junctions were formed at 2-3 mu m depth by a low-energy arsenic implant ( approximately 100 keV) which behaved as a finite diffusion source during the post-implant anneal. The best measured zero-bias resistance-area products (R0A) at 77 K were 1.67 Omega cm2 for the alloy layers for a cut-off wavelength of lambda c=14.94 mu m and 7.22 Omega cm2 for the interdiffused multiple layers for lambda c=15.92 mu m at 77 K. The results indicated a definite device performance improvement in double-layer compared to single-layer structures. Analysis of the transport properties for a device fabricated in a single layer with a cut-off wavelength of lambda c=8.35 mu m at 77 K and 9.1 mu m at 40 K suggested that the deviation from thermal processes occurs at temperatures of approximately 40 K.
    Semiconductor Science and Technology 12/1998; 5(3S):S45. · 1.92 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The critical steps in the development of MOVPE for the fabrication of MCT focal plane arrays (FPAs) are outlined. Improved purity of the source organometallics has enabled background donor concentrations in the mid 1014 cm-3 to be achieved, together with controlled back doping with donors or acceptors. Large area alloy uniformity is possible by the interdiffused multilayer process (IMP). However, large FPAs will need to be grown onto Si substrates to avoid thermal stress on the hybridized structure. Preliminary results are presented on a 256*256 element MWIR array. Results are also presented on optical in situ monitoring that will form the basis of improved epitaxial control in the future.
    Semiconductor Science and Technology 12/1998; 6(12C):C15. · 1.92 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The current-voltage characteristics and quantum efficiencies of double layer planar heterostructure photodiodes were investigated. Results are reported on devices with cutoff wavelengths of 1.8, 2.4, and 3.3 µm. For these respective devices, the dominant currents for temperatures >250,>200,>150K are diffusion currents limited by shallow Shockley-Hall-Read (SHR) processes. The remarkable result is that the electrical and optoelectronic properties of these devices of diverse cut-off wavelength can be explained by simple models using independently measured layer parameters such as the minority carrier lifetimes. For all three cases, the analysis suggests that the same shallow (SHR) centers located at 78% of the energy gap are causing the observed effects. These traps located in then-type base of the device are not influenced by the magnitude of n-type doping and this observation was used to significantly improve the performance of the devices and validate the predictive capability of the models used in the analysis. The shallow centers appear to be process induced rather than grown-in. This assertion is based on the observation that changes in the annealing process led to an order of magnitude improvement in the minority carrier lifetime.
    Journal of Electronic Materials 05/1998; 27(6):722-726. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The molecular beam epitaxy (MBE) growth technology is inherently flexible in its ability to change the Hg1−xCdxTe material’s bandgap within a growth run and from growth run to growth run. This bandgap engineering flexibility permits tailoring the device architecture to the various specific requirements. Material with active layer x values ranging from ∼0.198 to 0.570 have been grown and processed into detectors. This wide range in x values is perfectly suited for remote sensing applications, specifically the National Polar Orbiting Environmental Satellite System (NPOESS) program that requires imaging in a multitude of infrared spectral bands, ranging from the 1.58 to 1.64 µm VSWIR (very short wave infrared) band to the 11.5 to 12.5 µm LWIR (longwave infrared) band and beyond. These diverse spectral bands require high performance detectors, operating at two temperatures; detectors for the VSWIR band operate near room temperature while the SWIR, MWIR (mid wave infra red), LWIR and VLWIR (very long wave infrared) detectors operate near 100K, because of constraints imposed by the cooler for the NPOESS program. This paper uses material parameters to calculate theoretical detector performance for a range of x values. This theoretical detector performance is compared with median measured detector optical and electrical data. Measured detector optical and electrical data, combined with noise model estimates of ROIC performance are used to calculate signal to noise ratio (SNR), for each spectral band. The SNR are compared with respect to the meteorological NPOESS system derived focal plane. The derived system focal plane requirements for NPOESS are met in all the spectral bands.
    Journal of Electronic Materials 01/1997; 26(6):656-661. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Short wave infrared (SWIR) devices have been fabricated using Rockwell’s double layer planar heterostructure (DLPH) architecture with arsenic-ion implanted junctions. Molecular beam epitaxially grown HgCdTe/CdZnTe multilayer structures allowed the thin, tailored device geometries (typical active layer thickness was ∼3.5 µm and cap layer thickness was ∼0.4 µm) to be grown. A planar-mesa geometry that preserved the passivation advantages of the DLPH structure with enhanced optical collection improved the performance. Test detectors showed Band 7 detectors performing near the radiative limit (∼3-5X below theory). Band 5 detector performance was ∼4-50X lower than radiative limited performance, apparently due to Shockley-Hall-Read recombination. We have fabricated SWIR HgCdTe 256 × 12 × 2 arrays of 45 um × 45 µm detector on 45 µm × 60 µm centers and with cutoff wavelength which allows coverage of the Landsat Band 5 (1.5−1.75 µm) and Landsat Band 7 (2.08−2.35 µm) spectral regions. The hybridizable arrays have four subarrays, each having a different detector architecture. One of the Band 7 hybrids has demonstrated performance approaching the radiative theoretical limit for temperatures from 250 to 295K, consistent with test results. D* performance at 250K of the best subarray was high, with an operability of ∼99% at 1012 cm Hz1/2/W at a few mV bias. We have observed 1/f noise below 8E-17 AHz 1/2 at 1 Hz. Also for Band 7 test structures, Ge thin film diffractive microlenses fabricated directly on the back side of the CdZnTe substrate showed the ability to increase the effective collection area of small (nominally <20 µm µm) planar-mesa diodes to the microlens size of 48 urn. Using microlenses allows array performance to exceed 1-D theory up to a factor of 5.
    Journal of Electronic Materials 01/1997; 26(6):649-655. · 1.64 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Remote sensing applications including the National Polar Orbiting Environmental Satellite System (NPOESS) require imaging in a multitude of infrared spectral bands, ranging from the 1.58 micrometer to 1.64 micrometer VSWIR band to the 11.5 micrometer to 12.5 micrometer LWIR band and beyond. These diverse spectral bands require high performance detectors, operating over a range of temperatures; room temperature for the VSWIR band 100 K for MWIR, LWIR and VLWIR, these needs can all be met using molecular beam epitaxy (MBE) to grow HgCdTe. The flexibility inherent in the MBE growth technology is its ability to vary the HgCdTe material's bandgap within a growth run and from growth run to growth run, a capability necessary for remote sensing applications that require imaging in a wide variety of spectral bands. This bandgap engineering flexibility also permits tailoring the device architecture to the various specific system requirements. This paper combines measured detector optical and electrical data, with noise model estimates of ROIC performance to calculate signal to noise ratio (SNR), D* or noise equivalent temperature difference (NE(Delta) T), for each spectral band. The SNR, D* and/or NE(Delta) T are calculated with respect to system focal plane specifications, as required for the meteorological NPOESS.
    Proc SPIE 01/1997;
  • [Show abstract] [Hide abstract]
    ABSTRACT: This work presents characterization of implanted and annealed double layer planar heterostructure HgCdTe for p-on-n photovoltaic This work presents characterization of implanted and annealed double layer planar heterostructure HgCdTe for p-on-n photovoltaic devices. Our observation is that compositional redistribution in the structure during implantation/ annealing process differs devices. Our observation is that compositional redistribution in the structure during implantation/ annealing process differs from that expected from classical composition gradient driven interdiffusion and impacts the placement of the electrical junction from that expected from classical composition gradient driven interdiffusion and impacts the placement of the electrical junction with respect to the metallurgical heterointerface, which in turn affects quantum efficiency and RoA. The observed anomalous interdiffusion results in much wider cap layers with reduced composition difference between base with respect to the metallurgical heterointerface, which in turn affects quantum efficiency and RoA. The observed anomalous interdiffusion results in much wider cap layers with reduced composition difference between base and cap layer composition. The compositional redistribution can, however, be controlled by varying the material structure and cap layer composition. The compositional redistribution can, however, be controlled by varying the material structure parameters and the implant/anneal conditions. Examples are presented for dose and implanted species variation. A model is parameters and the implant/anneal conditions. Examples are presented for dose and implanted species variation. A model is proposed based on the fast diffusion in the irradiation induced damage region of the ion implantation. In addition, we demonstratetion induced damage region of the ion implantation. In addition, we demonstrate spatial uniformity obtained on molecular beam epitaxy (MBE) material of the compositional and implanted species profile. This spatial uniformity obtained on molecular beam epitaxy (MBE) material of the compositional and implanted species profile. This reflects spatial uniformity of the ion implantation/annealing Processes and of the MBE material characteristics. reflects spatial uniformity of the ion implantation/annealing Processes and of the MBE material characteristics.
    Journal of Electronic Materials 07/1996; 25(8):1312-1317. · 1.64 Impact Factor