Photo courtesy NOAO, taken with LBNL fully depleted CCD

Optical Characterization of 1.7 m NIR Detectors for SNAP

M. Brown (Michigan), J. Balleza (IDTL), N. Barron (Michigan), M. Borysow (Michigan), D. Figer

(IDTL), R. Foltz (DCL), B. Hill (DCL), S. Johnson (DCL), W. Lorenzon (Michigan), E. Morse (IDTL), N. Mostek (Indiana), S. Mufson (Indiana), M. Regan (IDTL), M. Schubnell (Michigan), R.Smith(Caltech), G. Tarle (Michigan), A. Tomasch (Michigan), A. Waczynski (DCL), for the SNAP Collaboration

AbstractUniversity of Michigan, Caltech, IDTL, JPL and DCL have been conducting an extensive program to characterize 1.7m detectors produced by Rockwell Scientific Company and by Raytheon Vision Systems with R&D funding from the SNAP program. Precision photometry at the ~2% level is a critical driver for the NIR detector system specifications for SNAP. A complete understanding of the quantum efficiency as a function of wavelength, time and temperature is an important element of the calibration of the NIR detector system. Raytheon HgCdTe detectors grown by Liquid Phase Epitaxy have excellent QE, which is flat from 900 to 1700nm without removing the CdZnTe substrate: 80% prior to anti-reflective coating, with 95% promised after applying an AR coating. The MBE HgCdTe grown by Rockwell achieves similar QE longward of 1300nm but rolls off at shorter wavelengths. Since SNAP’s diffraction limited images will be under-sampled, intra-pixel variation can have a significant impact on photometry. Therefore, we have scanned small sub-arrays with a custom made precision NIR spot projection system, and find that losses at the pixel boundaries are significantly greater for the Rockwell HgCdTe devices. SNAP continues to fund both manufacturers to improve their performance in the expectation of a large competitive procurement. Please see the companion poster on dark signals and noise.

All HgCdTe devices from Rockwell droop at short wavelengths. The improvement gained (in some cases) by removing the CdZnTe substrate suggests that this may be partially due to absorption in the CdZnTe substrate, however the Raytheon LPE process achieves higher QE without such substrate removal.

Cutoff Wavelength Shifts in Lattice Matched InGaAs

Arsenides, such as InGaAs, are known to have strongly temperature dependent band-gap Energies. The cutoff wavelength at the SNAP operating temperature of 140K is <1.6 m, if the room temperature cutoff is 1.7 m.

QE Variations near HgCdTe cutoff wavelengthQE and flat-field variations for wavelengths <1680 nm are ~ 3 to 4% for both HgCdTe manufacturers. Variations in the bandgap, and thus the cutoff, may become significant due to inconsistencies in the stoichiometry of the material. For Hg1-xCdxTe:

Eg = -0.302 + 1.93x – 0.810x2 + 0.832x3 – 5.35*10-4(1 - 2x)*T

A comparison of Raytheon and Rockwell HgCdTe @ 1720nm shows that the RVS material grown using LPE is much more uniform than the MBE growth used by Rockwell near the cutoff wavelength.

Measurements of SNAP prototype detectorsWe have tested Rockwell’s (RCS) HgCdTe, grown by Molecular Beam Epitaxy and hybridized to the HAWAII-2RG 2kx2k mux, and Raytheon’s (RVS) HgCdTe grown by Liquid Phase Epitaxy, hybridized to the SB-301 1kx1k (VIRGO) mux. Both are grown on 0.8mm thick CdZnTe substrates. InGaAs detectors grown on InP substrates by Sensors Unlimited have also been tested on the same muxes.

Quantum Efficiency

Intra-pixel Variations

Intra-pixel Variation ResultsThe plot below shows the integrated response in a 5x5 pixel region as a function of spot position for a 5m FWHM spot. The RVS response is uniform to better than 2% (left plot), while the Rockwell response varies up to 25% as the spot is moved within a given pixel. Uniform response within each pixel is essential for precision photometry without dithering for an under-sampled telescope such as SNAP.

The Spot-o-MaticThe Spot-o-Matic is a NIR pinhole projection system based on an optical LBNL design. The system projects diffraction limited spots ( = 1.4 m @ 1050 nm, 2.1 m @ 1550 nm) onto our NIR detectors. It is mounted on a precision XYZ stage, allowing automated focusing and pixel scans.

Light Guide Tube Lens

PinholeFilterXYZ Stage

MonitoringPhotodiode

ObjectiveLens

Beam Splitter

DewarWindow

X

Y

Z

Linear Encoder

RVS HgCdTe 141– Integrated Response RSC HgCdTe#40 – Integrated Response

RSC H2RG # 40– Single pixel response

The response for a single pixel as a function of spot position is shown on the right. The structure shown is typical of all pixels measured with the spot projection system at U. Michigan.

The sensitive pixel area extends beyond 18 microns, indicating a large amount of diffusion at the pixel boundary.

RVS HgCdTe #141 @ 1720nm RSC H2RG HgCdTe #40 @ 1720nm

Future development workThe detector tests described here were performed on engineering prototypes. R&D funding will continue to be supplied to both manufacturers to complete the optimization of the manufacturing process and to generate science grade arrays in the near future demonstrating all desirable characteristics simultaneously. The SNAP IR detector labs, in addition to testing these new devices as reported here, will investigate gain and sensitivity stability, limits on accuracy for conventional linearity correction, and the possible dependence of sensitivity on flux, which has been reported by R.Bohlin (STScI) based on HST data at ~1 m.

RSC

RSC, Substrate removed

"This work was supported by the Director, Office of Science, of the U.S. Department of Energy under Contract

No. DE AC03-76SF00098."