L. O. Bubulac

Stanford University, Palo Alto, California, United States

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Publications (65)119.25 Total impact

  • S J C Irvine · E R Gertner · L O Bubulac · R V Gil · D D Edwall ·
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    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. DOI:10.1088/0268-1242/6/12C/004 · 2.19 Impact Factor
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    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. DOI:10.1088/0268-1242/5/3S/011 · 2.19 Impact Factor
  • L O Bubulac · S J C Irvine · E R Gertner · J Bajaj · W P Lin · R Zucca ·
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    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. DOI:10.1088/0268-1242/8/1S/059 · 2.19 Impact Factor
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    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. DOI:10.1007/s11664-998-0043-1 · 1.80 Impact Factor
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    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.
    Proceedings of SPIE - The International Society for Optical Engineering 12/1997; 3221. DOI:10.1117/12.298105 · 0.20 Impact Factor
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    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. DOI:10.1007/s11664-997-0210-9 · 1.80 Impact Factor
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    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. DOI:10.1007/s11664-997-0211-8 · 1.80 Impact Factor
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    ABSTRACT: This paper describes recent advances in MBE HgCdTe technology. A new 3 inch production molecular beam epitaxy (MBE) system, Riber Model 32P, was installed at Rockwell in 1994. The growth technology developed over the years at Rockwell using the Riber 2300 R&D system was transferred to the 32P system in less than six months. This short period of technology transfer attests to our understanding of the MBE HgCdTe growth dynamics and the key growth parameters. Device quality material is being grown routinely in this new system. Further advances have been made to achieve better growth control. One of the biggest challenges in the growth of MBE HgCdTe is the day-to-day control of the substrate surface temperature at nucleation and during growth. This paper describes techniques that have led to growth temperature reproducibility within + - 1°C, and a variation in temperature during substrate rotation within 0.5°C. The rotation of the substrate during growth has improved the uniformity of the grown layers. The measured uniformity data on composition for a typical 3 cm × 3 cm MBE HgCdTe/CdZnTe shows the average and standard deviation values of 0.229 and 0.0006, respectively. Similarly, the average and standard deviation for the layer thickness are 7.5 and 0.06 µm, respectively. P-on-n LWIR test structure photodiodes fabricated using material grown by the new system and using rotation during growth have resulted in high-performance (R0)A, quantum efficiency) devices at 77 and 40K. In addition, 128 × 28 focal plane arrays with excellent performance and operability have been demonstrated.
    Journal of Electronic Materials 07/1996; 25(8):1394-1401. DOI:10.1007/BF02655040 · 1.80 Impact Factor
  • L. O. Bubulac · J. Bajaj · W. E. Tennant · M. Zandian · J. Pasko · W. V. Mc Levige ·
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    ABSTRACT: 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 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 R0A. 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 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 demonstrate 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.
    Journal of Electronic Materials 07/1996; 25(8):1312-1317. DOI:10.1007/BF02655025 · 1.80 Impact Factor
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    ABSTRACT: The objectives of this work are to study the physical and chemical structure of CdTe films using secondary ion mass spectrometry (SIMS) and atomic force miroscopy (AFM) and to demonstrate the usefulness of these analytical techniques in determining the characteristics of CdTe-passivation films deposited by different techniques on HgCdTe material. Three key aspects of CdTe passivation of HgCdTe are addressed by different analytical tools: a) morphological microstructure of CdTe films examined by atomic force microscopy; b) compositional profile across the interface determined by Matrix (Te)—SIMS technique; c) concentration of various impurities across the CdTe/HgCdTe structure profiled by secondary ion-mass spectrometry.
    Journal of Electronic Materials 08/1995; 24(9):1175-1182. DOI:10.1007/BF02653071 · 1.80 Impact Factor
  • M. Zandian · J. M. Arias · J. Bajaj · J. G. Pasko · L. O. Bubulac · R. E. Dewames ·
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    ABSTRACT: Characterization of defects in Hg1−xCdxTe compound semiconductor is essential to reduce intrinsic and the growth-induced extended defects which adversely affect the performance of devices fabricated in this material system. It is shown here that particulates at the substrate surface act as sites where void defects nucleate during Hg1−xCdxTe epitaxial growth by molecular beam epitaxy. In this study, we have investigated the effect of substrate surface preparation on formation of void defects and established a one-to-one correlation. A wafer cleaning procedure was developed to reduce the density of such defects to values below 200 cm−2. Focal plane arrays fabricated on low void density materials grown using this new substrate etching and cleaning procedure were found to have pixel operability above 98.0%.
    Journal of Electronic Materials 08/1995; 24(9):1207-1210. DOI:10.1007/BF02653075 · 1.80 Impact Factor
  • L. O. Bubulac · D. D. Edwall · S. J. C. Irvine · E. R. Gertner · S. H. Shin ·
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    ABSTRACT: Extrinsic p-type doping of long wavelength infrared-HgCdTe double layer heterostructure for p-on-n device application requires good control of the p-type dopant, regardless of the doping technique. The approach is to place the electrical junction ahead of the compositional interface, thus avoiding quantum efficiency reduction. This research addresses the As and P doping of HgCdTe by an implant/diffusion process. The data demonstrates an enhanced atomic diffusion process for As and P from an ion implanted source, within the single phase domain, with a diffusion rate orders of magnitude higher than the rate under Hg-saturated conditions at the same temperature. This work also reveals a new phenomenon, namely, a transition in the enhanced diffusion of both As and P from an exponential to a Gaussian redistribution. This transition is controlled by temperature at a given PHg. Gaussian diffusion dominates at high temperatures, T >400°C. The diffusion coefficient of the Gaussian mechanism decreases as the PHg increases, from Dp ~2 x 10-11 cm2/s at PHg ~0.02 atm to Dp ~3 x 10-14 cm2/ s under Hg-saturated conditions (quartz ampoule) at 440°C. The difference in the diffusion coefficients between open tube and closed tube (quartz ampoule), under nominally Hg-saturated conditions, indicates that PHg is undersaturated regardless of the Hg-source proximity. The deviation of PHg from saturation is estimated from the annealing furnace temperature profile up to a maximum of 50%. Variation of the diffusion coefficient close to Hg saturation appears to be sharply dependent on the actual PHg value (example: Dp ~1 x 10-12 cm2/s in open-tube anneal vs Dp ~3 x 10-14 cm2/s in closed tube at nominally the same temperature, T = 440°C). Comparative anneals of As and P showed faster diffusion rates for P than for As in both mechanisms.
    Journal of Electronic Materials 04/1995; 24(5):617-624. DOI:10.1007/BF02657970 · 1.80 Impact Factor
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    ABSTRACT: We have carried out a study and identified that MBE HgCdTe growth-induced void defects are detrimental to long wavelength infrared photodiode performance. These defects were induced during nucleation by having surface growth conditions deficient in Hg. Precise control and reproducibility of the CdZnTe surface temperature and beam fluxes are required to minimize such defects. Device quality material with void defect concentration values in the low 102 cm2 range were demonstrated.
    Journal of Electronic Materials 04/1995; 24(5):521-524. DOI:10.1007/BF02657957 · 1.80 Impact Factor
  • S. H. Shin · J. M. Arias · M. Zandian · J. G. Pasko · L. O. Bubulac · R. E. De Wames ·
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    ABSTRACT: Temperature and time dependent Hg-annealing studies for arsenic activation have been carried out on As-doped molecular beam epitaxy HgCdTe eitherin situ or by ion implantation to determine the extent of arsenic activation in the single layer. Enhanced As diffusion and activation in double layer heterostructures have also been investigated to further our understanding of the effects on zero bias resistance-area product (RoA) and quantum efficiency. The results show that the arsenic activation anneal is limited by Hg self-diffusion into the HgCdTe epilayer. Using this arsenic activation process for eitherin situ doped arsenic or implanted arsenic, high performance p-on-n double layer heterostructure photodiodes have been demonstrated on both mesa and planar device structures.
    Journal of Electronic Materials 01/1995; 24(5):609-615. DOI:10.1007/BF02657969 · 1.80 Impact Factor
  • S.J.C. Irvine · J. Bajaj · L.O. Bubulac ·
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    ABSTRACT: Recent progress in the growth of Hg1−xCdxTe (MCT) by metal organic vapor phase epitaxy (MOVPE) is reviewed. The preferred diode structure for LWIR detectors is the p/n heterostructure, which requires extrinsic doping of both n and p-type layers and good compositional control of the base and cap layers. Uniform n-type doping has been demonstrated using a new precursor, TIPIn, with Auger-limited lifetime down to a doping concentration of 1 × 1015 cm−3. p-type doping has been more difficult to control because Group V dopants can occupy either Group II or Group VI sites, leading to autodoping. Some encouraging progress has been made by doping under metal rich conditions. An alternative approach to p-type doping during growth is the Rockwell-developed process of As implantation, diffusion and activation annealing, which has been used to demonstrate near diffusion-limited LWIR diodes at 77K. Major strides in the reproducibility of the MOVPE process have been achieved by in situ monitoring. Laser reflectometry has been used to monitor growth rates and morphology throughout the growth of multiple layer structures. This wafer monitoring has been complemented by system monitoring, using Epison concentration monitors and pyrometry to measure temperature.
    MRS Online Proceeding Library 12/1993; 299. DOI:10.1557/PROC-299-99
  • S. J. C. Irvine · J. Bajaj · L. O. Bubulac · W. P. Lin · R. W. Gedridge · K. T. Higa ·
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    ABSTRACT: A new indium precursor, triisopropyl indium (TIPIn), has been used for doping MCT at low carrier concentrations. Previous attempts using indium organometallics resulted in a strong memory effect where residual doping would persist for many growth runs. Introducing TIPIn on the tellurium inject line resulted in a similarly strong memory doping but this was not observed when feeding the dopant in on the cadmium injection line. The TIPIn is believed to have been forming a low volatility adduct with diisopropyl tellurium (DIPTe) in the feed line and to have continued to evaporate at a low but significant rate. By keeping the TIPIn and DIPTe precursors separate until they entered the reactor, the desired low 1015 cm−3 carrier concentration and flat indium profiles could be achieved with good reproducibility. Good electrical characteristics were measured for these layers with Auger limited lifetime >1 μs at 77K.
    Journal of Electronic Materials 07/1993; 22(8):859-864. DOI:10.1007/BF02817498 · 1.80 Impact Factor
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    ABSTRACT: We recently succeeded in fabricating planar Hg1−yCdyTe/Hg1−xCdxTe (x<y) heterostructure photodiodes with the p-on-n configuration. Here we discuss early results in detail and present new results on an expanded range of infrared operation. The material used for this demonstration was grown by molecular beam epitaxy on lattice-matched CdZnTe substrates. The p-on-n planar devices consist of an arsenic-doped p-type epilayer (y∼0.28) atop a long wavelength infrared n-type epilayer (x=0.22–0.23). The planar junctions were formed by selective pocket diffusion of arsenic deposited on the surface by ion implantation. Detailed analysis of the current-voltage characteristics of these diodes as a function of temperature shows that they have high performance and that their dark currents are diffusion-limited down to 52K. Low frequency noise measurements at a reverse bias voltage of 50 mV resulted in noise current values (at 1 Hz) as low as 1×10−14 amps/Hz0.5 at 77K. Average RoA values greater than 106 Ω-cm2 at 40K were obtained for these devices with cut-off wavelength values in the 10.6 to 12 μm range. Seventy percent of these devices have RoA values greater than 105 Ω-cm2 at 40K; further studies are needed to improve device uniformity. These results represent the first demonstration that high performance long wavelength infrared devices operating at 40K can be made using HgCdTe material grown by a vapor phase epitaxy growth technique.
    Journal of Electronic Materials 07/1993; 22(8):1049-1053. DOI:10.1007/BF02817523 · 1.80 Impact Factor
  • J. Baars · D. Brink · D. D. Edwall · L. O. Bubulac ·
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    ABSTRACT: P-on-n mercury cadmium telluride (MCT) heterostructures grown by MOCVD with As and In as n- and p-type dopants, respectively, are examined by measuring the Seebeck and Hall coefficients between 20 and 320K. The results are analyzed regarding doping and composition of the layers by least squares fitting the experimental profiles with the calculated temperature dependencies. The electron and hole densities of the layers are calculated taking into account Fermi-Dirac statistics, a nonparabolic conduction band, a parabolic valence band, a discrete acceptor level, and fully ionized donors. For the Seebeck coefficient, the relation we previously showed to be valid for p-type MCT1 is used. This relation relies on the thermoelectric effect in a temperature gradient resulting from the diffusion of nondegenerate carriers scattered by LO-phonons. It also fits the observed thermoelectric properties of n-type MCT in a wide temperature range. The doping and structural parameters determined from the thermoelectric measurements agreed very well with As and In profiles obtained from secondary ion mass spectroscopy measurements and the data obtained from analyses of infrared transmission measurements.
    Journal of Electronic Materials 07/1993; 22(8):923-929. DOI:10.1007/BF02817506 · 1.80 Impact Factor
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    ABSTRACT: We report a process to fabricate planar Hg 1-y Cd y Te/Hg 1-x Cd x Te (x≪y) heterostructure photodiodes with the p‐on‐n configuration. The material used for this demonstration was grown by molecular beam epitaxy. The p‐on‐n planar devices consist of an arsenic‐doped p‐type epilayer (y=0.28) on top of a long wavelength infrared n‐type epilayer (x=0.225, λ=10 μm). The planar junctions were formed by selective pocket diffusion of arsenic deposited by ion implantation. The detailed analysis of the current‐voltage characteristics of these diodes as a function of temperature show that they have high performance and that their dark currents are diffusion limited down to 52 K. The results also show that the R 0 A values for these devices are highly uniform at 77 K.
    Applied Physics Letters 04/1993; 62(9-62):976 - 978. DOI:10.1063/1.108538 · 3.30 Impact Factor
  • S. H. Shin · J. M. Arias · M. Zandian · J. G. Pasko · L. O. Bubulac · R. E. De Wames ·
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    ABSTRACT: We report the results of annealing effects on the As-doped alloy HgCdTe grown by molecular beam epitaxy (MBE), arsenic (As) diffusion in HgCdTe from Hg-rich solutions at low temperatures, and As ion implantation at room temperature. Hall-effect measurements, secondary ion mass spectrometry and p-on-n test photodiodes were used to characterize the As activation. High As-doping levels (1017−1019 cm−3) could be obtained using either MBE growth, As diffusion or As ion-implantation. Annealed below 400°C, As doping in HgCdTe shows n-type characteristics, but above 410°C demonstrates that all methods of As doping exhibit p-type characteristics independent of As incorporation techniques. For example, for samples annealed at 436°C (PHg≈2 atm), in addition to p-type activation, we observe a significant improvement of p/n junction characteristics independent of the As source; i.e. As doping either in situ, by diffusion, or ion implantation. A study of this As activation of As-doped MBE HgCdTe as a function of anneal temperature reveals a striking similarity to results observed for As diffusion into HgCdTe and implanted As activation as a function of temperature. The observed dependence of As activation on partial pressure of Hg at various temperatures in the range of 250 to 450°C suggests that As acts as an acceptor at high Hg pressure (>1 atm) and as a donor at low Hg pressure (<1 atm) even under Hg-rich conditions.
    Journal of Electronic Materials 01/1993; 22(8):1039-1047. DOI:10.1007/BF02817522 · 1.80 Impact Factor