A G Steele

Dark current in quantum well infrared photodetectors

Modeling the dark current in quantum well infrared photodetectors has been a topic of much recent research, but the implications of many of the underlying assumptions have not been clarified. We attempt to justify one such model and to provide physical insight for its success. We compare the dark current expression with experiments on several samples, and show that the model provides a good approximation for a wide range of device parameters including barrier thicknesses from 250 to 700 A and number of wells from 4 to 32.

Importance of the upper state position in the performance of quantum well intersubband infrared detectors

We present the results of a systematic study of the dependence of intersubband infrared detector performance on the subband energies. By using samples cut from different positions on a single wafer grown with a known thickness variation it is possible to obtain a set of devices for which the well width varies in a controlled manner, but the barrier height remains fixed. Current versus voltage and responsivity measurements demonstrate the importance of the detector performance on the exact location of the upper state: optimum peak response is obtained when the subband is resonant with the top of barrier.

Influence of the number of wells in the performance of multiple quantum well intersubband infrared detectors

We present the results of a systematic study of a series of intersubband multiple quantum well detector samples having 4, 8, 16, and 32 wells. We find that while the absorption increases with the number of wells in the device, the detector current responsivity is insensitive to the number of wells.

Postgrowth tuning of quantum‐well infrared detectors by rapid thermal annealing

The peak detection wavelength of an operational quantum‐well infrared photodetector structure has been red shifted using rapid thermal annealing to partially intermix the well and barrier layers. Successive anneals at 850 °C were used to tune an 8.13 μm detector continuously out to 9.13 μm. All of the fabricated detectors were operational in spite of very long annealing times of up to 300 s. The peak spectral responsivity at a device current of 10 μA dropped from 0.62 to 0.12 A/W after the longest anneal time, but the broadband responsivity only dropped by a factor of 3 due to a simultaneous increase in the detection spectral bandwidth.

Isotopic and other influences on the realization of the triple point of hydrogen

Within an international collaboration of the eight metrological institutes represented by the authors, the dependence of the triple-point temperature of equilibrium hydrogen on the deuterium content at low concentrations has been precisely determined so that the uncertainty in realizing the triple point as a temperature fixed point might be reduced by nearly one order of magnitude. To investigate the thermodynamic properties of the hydrogen–deuterium mixtures and to elucidate the factors that influence the melting temperature, 28 sealed fixed-point cells have been filled and measured, and some of these have been compared with an open-cell system. Hydrogen gas with a deuterium content ranging from 27.2 µmol D/mol H to 154.9 µmol D/mol H was studied using cells containing five different types of spin-conversion catalyst, with different catalyst-to-liquid volume ratios (a few per cent to more than 100%) and of different designs. The latter consideration is especially influential in determining the thermal behaviour of the cells and, thus, the temperature-measurement errors. The cells were measured at the eight participating institutes in accordance with a detailed protocol that facilitates a direct comparison of the results. Through analysis of the measurements, significant inter-institute deviations due to different measurement facilities and methods have been ruled out with respect to the determination of both the melting temperatures and the thermal parameters of the cells. The uncertainty estimates for the determination of the deuterium content have been verified by including isotopic analysis results from four different sources. The slope of the dependence of the triple-point temperature of equilibrium hydrogen isotopic mixtures on the deuterium content has been deduced from the melting temperatures of those sample portions not in direct contact with the catalysts. Evaluation of the data using different mathematical methods has yielded an average value of 5.42 µK per µmol D/mol H, with an upper bound of the standard uncertainty of 0.31 µK per µmol D/mol H. This is close to the literature value of 5.6 µK per µmol D/mol H that was obtained at higher deuterium concentrations. (Some figures in this article are in colour only in the electronic version)

CCT-K2: key comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K

Calibrated capsule-type standard platinum resistance thermometers were used to compare national realizations of the International Temperature Scale of 1990 (ITS-90) from 13.8033 K, the triple point of equilibrium hydrogen, to 273.16 K, the triple point of water, for seven countries in CIPM Key Comparison CCT-K2. Measurements were made at temperatures close to the eight low-temperature defining fixed points of the ITS-90, using a copper comparison block capable of simultaneously holding nine thermometers. Two separate measurement runs were performed, allowing two different groups of capsules from each laboratory to be examined. The results are used to determine the degree of equivalence of the independent national realizations of the scale for use in the Mutual Recognition Arrangement Appendix B database. In addition, measurements were made with the first group of thermometers at approximately eighty temperatures throughout the cryogenic range, which provide information to evaluate some of the so-called scale non-uniqueness issues inherent in the ITS-90 interpolation scheme.

Low dark current dual band infrared photodetector using thin AlAs barriers and Γ‐X mixed intersubband transition in GaAs quantum wells

A low dark current dual band quantum well infrared photodetector is demonstrated by adding thin AlAs barriers to the usual detector structure, which consists of Si‐doped GaAs wells separated by thick AlGaAs barriers. The advantages of adding the thin AlAs barriers to clad the quantum wells are that (a) the detector displays a low dark current and (b) intersubband photocurrents result from transitions from both the Γ ground to the first excited state, and from the Γ ground to a mixed Γ‐X excited state because the X‐valley band edge forms a well in AlAs and intrinsic Γ‐X mixing occurs. The spectral peaks of these two transitions, which occur at 8.5 and 5.5 μm in our test structure, can be varied by changing device parameters during growth.

Recent recommendations of the Consultative Committee for Length (CCL) regarding strategies for evaluating key comparison data

A workshop on statistical techniques for evaluating key comparison data in length metrology was held as part of the September 2005 meetings of the Working Group on Dimensional Metrology of the Consultative Committee for Length (CCL) of the International Committee for Weights and Measures. This paper summarizes the discussion at this workshop and the resulting recommendations, subsequently adopted by the CCL, for evaluating future key comparison data in dimensional metrology. Motivation and supporting information relating to the recommendations are included.

Pair-difference chi-squared statistics for Key Comparisons

Pair-difference chi-squared statistics are useful for analysing metrological consistency within a Key Comparison. We show how they relate to classical chi-squareds and how they can be used with full rigour, for any comparison of a scalar measurand, to compare the observed dispersion of results with the dispersion that would be expected on the basis of the claimed uncertainties. In several limits, the distributions of these pair-difference statistics are exact chi-squareds. For other cases, the Monte Carlo method can evaluate the distributions even in the presence of non-Gaussian uncertainty distributions, including the Student distribution to be construed when a participant has reported a degrees of freedom. Monte Carlo methods also treat inter-laboratory covariances in a transparent manner appropriate for metrology. Pair-difference chi-squared statistics are independent of the choice of a Key Comparison Reference Value (KCRV) and so may expedite the process of analysis, consensus building and publication for Key Comparisons. They are appropriate for judging pair metrology. We discuss them as a necessary, but not sufficient, test for a Key Comparison reported in the conventional way using a KCRV. The deficiencies of using solely the classical chi-squared test are discussed. A good remedy is available with pair-difference chi-squared statistics.

Chi-squared statistics for KCRV candidates

We examine chi-squared statistics that are appropriate for analysing the adequacy of different key comparison reference value (KCRV) candidates in accounting for the observed dispersion of results of a key comparison, about the candidate estimator and within the stated uncertainty claims. We extend the analysis to cover cases where the uncertainty budgets incorporate low degrees of freedom or have significant correlations. In this context, we discuss when it is important to view the KCRV as a method and not merely as a number. To use these statistics for the usual chi-squared tests of consistency, the required distributions (that can depart from the exact chi-squared distribution) can readily be evaluated by Monte Carlo simulation, for any KCRV algorithm that uses only the peer results of the comparison. Similarly, the effects of non-Gaussian distributions (such as the Student distribution implied when a participant has reported finite degrees of freedom) can be evaluated with the requisite precision.