We present a novel method of stabilizing a distributed-feedback laser. It’s the first time to our knowledge that the timemeasurement technique is used in laser frequency stabilization. We obtain the laser frequency deviation information from the Fabry-Perot interferometer based on the pulse delay time. In contrast to traditional approaches, the laser can be stabilized in the quasi-continuous spectrum that the interferometer covering. Our method can obtain the error signal from a high signal to noise ratio (SNR) of the voltage signal and not limited by the frequency references. It also avoids many traditional problems, such as power insensitive, modulation, low-level signal, and finite frequency references. A relative frequency fluctuation less than 0.1 MHz is achieved and the root of an Allan variance is about 10-11 for an average time of 10 s.
Surface defects of molecular beam epitaxially grown HgCdTe are the major concern in developing large format
infrared focal plane arrays. Voids were usually observed on the HgCdTe surfaces as previously reported, they were
originated either from the improper substrates preparation or from the growth condition. However, the defects formation
with impurities has not been addressed. This paper presents our recent observation on defects induced by the impurities
involved in the mercury beam fluxes. These defects can be craters or bumps, having a spatially clustering feature. To
identify the origin of these kinds of defects, experiments were performed on HgCdTe as well as CdTe with mercury flux,
and the defects were observed and analyzed by using SEM and EDAX. The result, for the first time, confirmed that
impurities in the mercury beam were responsible to the formation of surface defects.
This paper presents the recent progress on the study of device processings at multilayer HgCdTe film for integrated two-color (SWIR/MWIR) n-p-P-P-N detector arrays. The four-layer p-P-P-N heterostructures Hg1-xCdxTe film needed to achieve two color detector arrays was grown by molecular beam epitaxy (MBE) on (211)B oriented GaAs substrates. The secondary ion mass spectroscopy (SIMS) data for the HgCdTe film was obtained. The p-type layer on top of a thin P-type potential barrier layer and the SWIR P-on-N homojunction photodiode formed in-situ during MBE growth using indium impurity doping was processed into the MWIR planar photodiode by selective B+-implantation. The preliminary 256×1 linear arrays of SWIR/MWIR HgCdTe two-color FPAs detector were then achieved by mesa isolation, side-wall passivation and contact metallization. At 78K, the average R0A values of SWIR and MWIR are 3.852×105 Wcm2 and 3.015×102 Wcm2, and the average peak detectivities Dλp* are 1.57×1011cmHz1/2/W and 5.63×1010 cmHz1/2/W respectively. The SWIR photodiode cut-off wavelength is 3.04μm and the MWIR photodiode cut-off wavelength is 5.74μm, quite consistent with the initial device design. The SWIR response spectrum of the two-color detector with a distinct fall-off at shorter wavelength regime was discussed especially.
CdTe growth on Si is the major challenge for HgCdTe. The recent results on MBE growth of 3-in CdTe(211)B/Si are reported. The Si substrates were (211) orientated, and a low temperature surface cleaning process was employed. To obtain twin-free CdTe(211)B, nucleation process of ZnTe on Si was studied at different conditions. Under the optimal growth condition, the average FWHM value less than 120 arc sec of twin-free CdTe(211)B films for 10-12μm was obtained. The lowest FWHM value of 100 arc sec was achieved.
Some results on the arsenic incorporation in MBE-grown HgCdTe are described. It was found that arsenic surface sticking coefficient during HgCdTe growth was very low, ~1x10-4 at 170°C being very sensitive to the growth temperature. The annealing experiments for activation of arsenic were performed, and the importance of the ambient mercury for the arsenic site transfer was confirmed. It was found that the arsenic dopants could be fully activated as acceptors by anneals at 285°C under the mercury-saturated pressure. The activation energy for the isolated arsenic acceptors in HgCdTe (x=0.27~0.33) was determined as to be 19.5 meV, which decreases with (Na-Nd)1/3 at a slop of 3.1x10-5 meV cm . The diffusion coefficient of arsenic in HgCdTe of 1.0±0.9x10-16 cm2/sec, 8±3x10-15 cm2/sec and 1.5±0.9x10-13 cm2/sec were obtained at annealing temperatures of 240°C, 380°C and 440°C under mercury saturated pressure, respectively.
This paper describes some recent results on surface defects, uniformity, dislocation density as well as device applications of MBE growth of HgCdTe at the research center of advanced materials and devices. The features of different surface defects and their origins were studied by using SEM/EDX observations on HgCdTe epilayers with different growth conditions. A variety of surface defects was observed and the formation mechanism was discussed. A good uniformity was observed over 3-in HgCdTe wafers, the Stddev/mean in x and thickness were 1.2%, and 2.7%, respectively. It was found that the dislocation density was sensitive to growth parameters and the composition. The ZnCdTe substrates with 4% mole fraction were found to be suitable for LW HgCdTe, however, for the HgCdTe of shorter wavelengths different Zn composition is required. An average value of EPD of 4.2×105cm−2 was obtained for LW samples. The MBE grown HgCdTe were incorporated into some preliminary FPA devices.