S.A. Ringel

Publications (90)158.62 Total impact

• Article: Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

  Emre Gur, G. Tabares, A. Arehart, J. M. Chauveau, A. Hierro, S. A. Ringel
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  ABSTRACT: Deep level defects in n-type unintentionally doped a-plane MgxZn1-xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1-xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec - 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec - 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec - 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec - 1.4 eV and Ec - 2.6 eV levels in Mg alloyed samples.
  Journal of Applied Physics 12/2012; 112:123709. · 2.17 Impact Factor
• Article: Spectrum-optimized Si-based III-V multijunction photovoltaics

  T J Grassman, A M Carlin, J Grandal, C Ratcliff, L Yang, M J Mills, S A Ringel
  Proc. of SPIE Vol. 01/2012; 8256:82560R-1.
• Conference Proceeding: Lattice-matched GaP/SiGe virtual substrates for low-dislocation density GaInP/GaAsP/Si solar cells

  A M Carlin, T J Grassman, M R Brenner, J Grandal, C Ratcliff, L Yang, M Mills, P Sharma, E A Fitzgerald, S A Ringel
  Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
• Conference Proceeding: Growth and characterization of InGaAs quantum dots on metamorphic GaAsP templates by molecular beam epitaxy

  J Grandal, T J Grassman, A M Carlin, M R Brenner, B Galiana, J A Carlin, L Yang, M J Mills, S A Ringel
  Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
• Article: Optically-aligned visible/near-infrared dual-band photodetector materials and devices on GaAs using metamorphic epitaxy

  K. Swaminathan, T. J. Grassman, L.-M. Yang, Q. Gu, M. J. Mills, S. A. Ringel
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  ABSTRACT: A monolithic, epitaxially-integrated, vertically-aligned, multi-band photodetector architecture has been demonstrated via the successful growth and fabrication of metamorphic back-to-back n-i-p/p-i-n In0.61Ga0.39P/In0.14Ga0.86As visible/near-IR dual-detector devices. The back-to-back diode design enables simultaneous and independent operation of detectors in both bands with low optical cross talk (<−10 dB outside the 690–720 nm range) and complete electrical isolation between the sub-detectors. The high electronic quality of the resultant metamorphic materials was confirmed via deep level transient spectroscopy, which revealed total trap concentrations of 5 × 1012 cm−3 for the In0.14Ga0.86As and 2 × 1014 cm−3 for the In0.61Ga0.39P sub-detectors, enabling low, room temperature reverse bias (−2 V) dark current densities of 4 × 10−8 A cm−2 and 7 × 10−12 A cm−2, respectively. High responsivity and specific detectivity values, at a working bias of −2 V, were measured: 0.41 A/W and 8.6 × 1011 cm Hz1/2/W for the In0.14Ga0.86As sub-detectors (at 980 nm), and 0.30 A/W and 2.0 × 1014 cm Hz1/2/W for the In0.61Ga0.39P sub-detectors (at 680 nm). The successful integration of high-quality lattice-mismatched materials, combined with the excellent sub-detector performances, demonstrate the potential for extending such a multi-band photodetector technology to achieve simultaneous detection of a wide range of wavelength bands with tunable cut-off wavelengths.
  Journal of Applied Physics 09/2011; 110(6):063109-063109-8. · 2.17 Impact Factor
• Article: Next generation defect characterization in nitride HEMTs

  A. R. Arehart, A. C. Malonis, C. Poblenz, Y. Pei, J. S. Speck, U. K. Mishra, S. A. Ringel
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  ABSTRACT: A series of constant drain current deep level transient and optical spectroscopies (CID-DLTS/DLOS) is described, which allows characterization of deep levels with lateral spatial resolution in fully fabricated high electron mobility transistors (HEMTs). The techniques are used to examine the role of SiNx passivation in the access region of AlGaN/GaN HEMTs, in part to verify lateral resolution capabilities. Two distinct trap spectra were observed correlating to the under-gate and access regions of the HEMT. Traps in the access region had energy levels of of EC-0.43, EC-1.3, EC-2.3, and EC-3.7 eV. The impact of each access region trap on the drain resistance before and after passivation was measured, with the total transient drain resistance change being reduced from 0.47 Ω-mm to 0.11 Ω-mm as a result of their partial passivation by SiNx, resulting in improved device performance. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  physica status solidi (c) 05/2011; 8(7‐8):2242 - 2244.
• Article: Metamorphic In(0.20)Ga(0.80)As p-i-n photodetectors grown on GaAs substrates for near infrared applications.

  K Swaminathan, L-M Yang, T J Grassman, G Tabares, A Guzman, A Hierro, M J Mills, S A Ringel
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  ABSTRACT: The growth and performance of top-illuminated metamorphic In(0.20)Ga(0.80)As p-i-n photodetectors grown on GaAs substrates using a step-graded In(x)Ga(1-x)As buffer is reported. The p-i-n photodetectors display a low room-temperature reverse bias dark current density of ~1.4×10(-7) A/cm(2) at -2 V. Responsivity and specific detectivity values of 0.72 A/W, 2.3×10(12) cm·Hz(1/2)/W and 0.69 A/W, 2.2×10(12) cm·Hz(1/2)/W are achieved for Yb:YAG (1030 nm) and Nd:YAG (1064 nm) laser wavelengths at -2 V, respectively. A high theoretical bandwidth-responsivity product of 0.21 GHz·A/W was estimated at 1064 nm. Device performance metrics for these GaAs substrate-based detectors compare favorably with those based on InP technology due to the close tuning of the detector bandgap to the target wavelengths, despite the presence of a residual threading dislocation density. This work demonstrates the great potential for high performance metamorphic near-infrared InGaAs detectors with optimally tuned bandgaps, which can be grown on GaAs substrates, for a wide variety of applications.
  Optics Express 04/2011; 19(8):7280-8. · 3.59 Impact Factor
• Conference Proceeding: Spatially-discriminating trap characterization methods for HEMTs and their application to RF-stressed AlGaN/GaN HEMTs

  A.R. Arehart, A. Sasikumar, G.D. Via, B. Winningham, B. Poling, E. Heller, S.A. Ringel
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  ABSTRACT: New constant drain-current deep level optical/transient spectroscopy (CI_D-DLTS/DLOS) methods to quantify trap energies and concentrations in AlGaN/GaN high electron mobility transistors (HEMTs) are described. These methods are applied to RF stressed HEMTs to characterize the impact of stressing on traps and identified a significant increase in virtual gate related levels.
  Electron Devices Meeting (IEDM), 2010 IEEE International; 01/2011
• Conference Proceeding: Expanding the palette: Metamorphic strategies over multiple lattice constant ranges for extending the spectrum of accessible photovoltaic materials

  T J Grassman, A M Carlin, K Swaminathan, C Ratcliff, J Grandal, L Yang, M J Mills, S A Ringel
  Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE; 01/2011
• Conference Proceeding: Traps in AlGaInP materials and devices lattice matched to GaAs for multi-junction solar cells

  A.R. Arehart, M.R. Brenner, Z. Zhang, K. Swaminathan, S.A. Ringel
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  ABSTRACT: Deep levels in solid-source MBE-grown n- and p-type (Al_0.09Ga_0.91)_0.51In_0.49P are investigated using deep level transient spectroscopy (DLTS). These results are correlated with background oxygen impurities measured by secondary ion mass spectroscopy and electrical properties using Hall effect. Oxygen impurity concentration is found to depend weakly on substrate offcut conditions in MBE-grown AlGaInP films. This is used to investigate the role of oxygen on deep levels in the n- and p-type samples using (100) GaAs substrates with three different substrate offcut conditions (A, B, and C). The DLTS of n-type AlGaInP reveals deep levels at E_C-0.22, E_C-0.31, E_C-0.69 eV and E_C-1.0 eV. The E_C-0.69 eV concentration tracked oxygen incorporation while the other levels decreased while the oxygen incorporation increased indicating possible secondary offcut effects. In general, we find a direct correlation between reduced carrier compensation, increased carrier mobility, lower trap concentration and lower oxygen content as a function of systematic changes in substrate offcut conditions.
  Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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  Available from: 144.206.159.178

  Article: Impact of N- and Ga-face polarity on the incorporation of deep levels in n-type GaN grown by molecular beam epitaxy

  A. R. Arehart, T. Homan, M. H. Wong, C. Poblenz, J. S. Speck, S. A. Ringel
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  ABSTRACT: Deep levels in N-face and Ga-face n-type GaN grown by plasma-assisted molecular beam epitaxy were detected, analyzed and compared using deep level optical spectroscopy (DLOS) and conventional thermal deep level transient spectroscopy (DLTS), which together enable deep level detection throughout the GaN band gap. A redistribution of band gap states was observed between the two GaN crystal growth polarities but with a similar total trap density. Most significant was a tenfold concentration increase in a trap at EC-0.25 eV that is likely related to nitrogen vacancies for the N-face polarity material, with no significant change for the Ga-vacancy-related level at EC-2.60 eV. The DLOS results suggest that carbon impurities, which generate several GaN band gap states, appear to incorporate differently for both crystal polarities with the potential carbon interstitial at EC-1.28 eV being undetected for N-face material. Finally, low concentrations of several new levels in the N-face n-GaN not previously observed in Ga-face n-GaN were observed.
  Applied Physics Letters 06/2010; 96(24):242112-242112-3. · 3.84 Impact Factor
• Article: Effect of nitrogen plasma power on defect levels in Ni/n-GaN Schottky diodes grown by molecular beam epitaxy

  A. R. Arehart, C. Poblenz, J. S. Speck, S. A. Ringel
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  ABSTRACT: The incorporation of deep level defects in n -type GaN grown by plasma assisted molecular beam epitaxy was studied via systematic adjustment of the nitrogen plasma source power from 150 to 400 W while maintaining a constant V/III beam flux ratio. Deep level optical spectroscopy and conventional thermal deep level transient spectroscopy measurements, which together enable deep level detection throughout the GaN band gap, revealed several deep level concentrations that depend strongly on rf-plasma power. The concentrations of the gallium vacancy deep level at E _C-2.60 eV and carbon-related point defects with energy levels at E _C-3.28 and E _C-1.35 eV are found to be very sensitive to the nitrogen source power, increasing by up to 50 times for a corresponding increase in plasma power from 150 to 400 W. The relation between the concentrations of these traps and plasma power follows an Arrhenius-type behavior and is suggestive of plasma damage associated with the energetics of the constituent active nitrogen species. In contrast, two traps at E _C-0.86 and E _C-0.59 eV did not exhibit a systematic dependence on plasma power, with this difference a result of the dislocation-related nature of these defects.
  Journal of Applied Physics 04/2010; · 2.17 Impact Factor
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  Available from: vt.edu

  Article: Strain relaxation properties of InAsyP1-y metamorphic materials grown on InP substrates

  Mantu K. Hudait, Y. Lin, S. A. Ringel
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  ABSTRACT: The strain relaxation mechanism and defect properties of compositionally step-graded InAs _y P _1-y buffers grown by molecular beam epitaxy on InP have been investigated. InAsP layers having lattice misfits ranging from 1% to 1.4% with respect to InP, as well as subsequently grown lattice matched In _0.69 Ga _0.31 As overlayers on the metamorphic buffers were explored on both (100) and 2° offcut (100) InP substrates. The metamorphic graded buffers revealed very efficient relaxation coupled with low threading dislocation densities on the order of (1–2)×10⁶ cm ^-2 for the range of misfit values explored here. A detailed analysis via high resolution x-ray diffraction revealed that the strain relaxed symmetrically, with equivalent numbers of α and β dislocations, and to greater than 90% for all cases, regardless of substrate offcut. Further analysis showed the relaxation to always be glide limited in these materials when grown on a graded buffer compared to a single step layer. The threading dislocation density was observed by plan-view transmission electron microscopy to be constant for the range of misfit values studied here in the top layer of the graded structures, which is attributed to the very efficient use of residual dislocations and the dominance of dislocation glide over nucleation in these graded anion metamorphic buffers, suggesting great promise for metamorphic devices with lattice constants greater than that of InP to be enabled by InAsP metamorphic structures on InP.
  Journal of Applied Physics 04/2009; · 2.17 Impact Factor
• Article: Deep level optical and thermal spectroscopy of traps in n-GaN grown by ammonia molecular beam epitaxy

  A. R. Arehart, A. Corrion, C. Poblenz, J. S. Speck, U. K. Mishra, S. A. Ringel
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  ABSTRACT: The incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy (MBE) is studied via systematic adjustment of the NH3/Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH3/Ga flux ratio. A general trend of lower concentration for deep levels at EC−3.28, EC−1.28, EC−0.62, and EC−0.25 eV with higher NH3/Ga flux ratio was observed, with the strongest reduction at the EC−0.25 eV level, consistent with expectations for a VN-related defect. The known CN impurity state at EC−3.28 eV and suspected CI-related state at EC−1.28 eV also showed a moderate decrease in concentration at the higher NH3/Ga flux ratio. In contrast, the VGa-related defect at EC−2.62 eV was insensitive to the NH3/Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios suggestive of independent physical sources. However, the total trap concentrations were significantly reduced for higher NH3/Ga flux ratios in n-type GaN grown by ammonia-MBE under the range of growth conditions used in this study, suggesting that higher NH3/Ga flux ratios will generate higher electronic quality GaN material when using ammonia-based MBE for device applications.
  Applied Physics Letters 09/2008; 93(11):112101-112101-3. · 3.84 Impact Factor
• Article: Separation of bulk and surface electron transport in metamorphic InAs layers using quantitative mobility spectrum analysis

  Y. Lin, A. R. Arehart, A. M. Carlin, S. A. Ringel
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  ABSTRACT: Electron transport in low dislocation density, strain-relaxed InAs layers grown on metamorphic In As _y P _1-y/ In P substrates by molecular beam epitaxy was characterized using quantitative mobility spectrum analysis (QMSA) of Hall effect measurements. QMSA applied to systematically varied metamorphic InAs samples reveals high bulk electron mobilities of ∼20 000 cm ²/ V s at 300 K at a Si doping concentration of 1×10¹⁷ cm ^-3 , simultaneously with a separate population of much slower electrons having an average mobility of ∼2400 cm ²/ V s due to parallel conduction within the InAs surface electron accumulation layer. Measurements made on higher doped samples reveal only a single electron population participating in transport due to lowered surface band bending that reduces surface accumulation of electrons in conjunction with the high conductivity of the high mobility metamorphic InAs bulk that overwhelms any remaining surface conductivity in the Hall effect measurements.
  Applied Physics Letters 09/2008; · 3.84 Impact Factor
• Article: Comparison of deep level incorporation in ammonia and rf‐plasma assisted molecular beam epitaxy n‐GaN films

  A. R. Arehart, A. Corrion, C. Poblenz, J. S. Speck, U. K. Mishra, S. P. DenBaars, S. A. Ringel
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  ABSTRACT: The use of rf-plasma and ammonia nitrogen sources for growth of GaN films by molecular beam epitaxy (MBE) are compared in terms of defect incorporation using deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS). To better improve the understanding of ammonia-based MBE growth of GaN and potential defect sources as opposed to the more studied plasma source-based MBE-grown material several V/III ratios were also investigated, which were generated via systematic adjustment of the ammonia flow rates during growth. The DLOS spectra, comparing deep traps within the n-GaN grown using N-plasma and ammonia sources, reveal the presence of the same deep levels due to background carbon and gallium vacancies, with energy levels at EC-3.28, EC-2.62, and EC-1.28. The DLTS results of the N-plasma and ammonia-based MBE samples show two similarly dominant electron traps at EC-0.60, and EC-0.24 in each sample. Measurements made as a function of V/III flux ratio for ammonia-based MBE growth indicate a large dependence of the EC-0.24 eV trap concentration on growth flux ratio, which is significant for guiding continued optimization of this promising MBE growth method for GaN devices. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  physica status solidi (c) 04/2008; 5(6):1750 - 1752.
• Article: Characterization of majority and minority carrier deep levels in p-type GaN:Mg grown by molecular beam epitaxy using deep level optical spectroscopy

  A. Armstrong, J. Caudill, A. Corrion, C. Poblenz, U. K. Mishra, J. S. Speck, S. A. Ringel
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  ABSTRACT: Deep level defects in p-type GaN:Mg grown by molecular beam epitaxy were characterized using steady-state photocapacitance and deep level optical spectroscopy (DLOS). Low frequency capacitance measurements were used to alleviate dispersion effects stemming from the deep Mg acceptor. Use of DLOS enabled a quantitative survey of both deep acceptor and deep donor levels, the latter being particularly important due to the limited understanding of minority carrier states for p-type GaN. Simultaneous electron and hole photoemissions resulted in a convoluted deep level spectrum that was decoupled by emphasizing either majority or minority carrier optical emission through control of the thermal filling time conditions. In this manner, DLOS was able to resolve and quantify the properties of deep levels residing near both the conduction and valence bandedges in the same sample. Bandgap states through hole photoemission were observed at Ev+3.05 eV, Ev+3.22 eV and Ev+3.26 eV. Additionally, DLOS revealed levels at Ec−3.24 eV and Ec−2.97 eV through electron emission to the conduction band with the former attributed to the Mg acceptor itself. The detected deep donor concentration is less than 2% of activated [Mg] and demonstrates the excellent quality of the film.
  Journal of Applied Physics 03/2008; 103(6):063722-063722-6. · 2.17 Impact Factor
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  Available from: vt.edu

  Conference Proceeding: 80nm In0.52Al0.48As/In0.53Ga0.47As/InAs0.3P0.7 Composite channel HEMT with an fT of 280GHz

  D. Liu, M. Hudait, Y. Lin, S.A. Ringel, W. Lu
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  ABSTRACT: We have demonstrated a high performance 80nm gate In_0.52Al_0.48As/In_0.53Ga_0.47As/InAs_0.3P_0.7 composite channel HEMT. The device has a g_m as high as 1 S/mm and an f_T 280 GHz. To complete this study, the noise and power performance of such device will be further studied and compared with conventional InGaAs channel devices in near future.
  Semiconductor Device Research Symposium, 2007 International; 01/2008
• Article: High-mobility two-dimensional electron gas in InAlAs/InAs heterostructures grown on virtual InAs substrates by molecular-beam epitaxy

  Y. Lin, J. A. Carlin, A. R. Arehart, A. M. Carlin, S. A. Ringel
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  ABSTRACT: In _0.8 Al _0.2 As / In As heterostructures were grown on virtual InAs substrates consisting of a relaxed In As _y P _1-y step-graded buffer grown on InP by molecular-beam epitaxy. Hall measurements revealed the presence of a high-mobility two-dimensional electron gas within the relaxed InAs layer, with a peak electron mobility of 133 000 cm ²/ V s at 25 K . In contrast, identical In Al As / In As heterostructures grown directly on InAs buffers on InP showed only bulk transport characteristics. A combination of transport modeling and electron microscopy demonstrates that reduced dislocation scattering in the channel region is responsible for observing the two-dimensional transport within the relaxed InAs on graded InAsP. These results demonstrate the potential of achieving ultrahigh-speed InAs based high electron mobility transistors using relaxed, virtual InAs substrates on InP.
  Applied Physics Letters 02/2007; · 3.84 Impact Factor
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  Available from: escholarship.org

  Article: Quantitative observation and discrimination of AlGaN- and GaN-related deep levels in AlGaN/GaN heterostructures using capacitance deep level optical spectroscopy

  A. Armstrong, A. Chakraborty, J. S. Speck, S. P. DenBaars, U. K. Mishra, S. A. Ringel
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  ABSTRACT: Deep levels were observed using capacitance deep level optical spectroscopy (DLOS) in an AlGaN/GaN heterostructure equivalent to that of a heterojunction field effect transistor. Band gap states were assigned to either the AlGaN or GaN regions by comparing the DLOS spectra in accumulation and pinch-off modes, where the former reflects both AlGaN- and GaN-related defects, and the latter emphasizes defects residing in the GaN. A band gap state at Ec−3.85 eV was unambiguously identified with the AlGaN region, and deep levels at Ec−2.64 eV and Ec−3.30 eV were associated with the GaN layers. Both the AlGaN and GaN layers exhibited additional deep levels with large lattice relaxation. The influence of deep levels on the two-dimensional electron gas sheet charge was estimated using a lighted capacitance-voltage method.
  Applied Physics Letters 12/2006; 89(26):262116-262116-3. · 3.84 Impact Factor