Imaging One-Dimensional and Two-Dimensional Planar Photodiode Detectors Fabricated by Ion Milling Molecular Beam Epitaxy CdHgTe

Journal of Electronic Materials, Jun 2005 by Haakenaasen, R, Steen, H, Selvig, E, Lorentzen, T, Et al

Imaging one-dimensional (1-D) and two-dimensional (2-D) arrays of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) planar photodiodes were fabricated by ion milling of vacancy-doped molecular beam epitaxy Cd^sub x^Hg^sub 1-x^Te layers. Sixty-four-element 1-D arrays of 26 × 26 µm^sup 2^ or 26 × 56 µm^sup 2^ diodes were processed. Zero-bias resistance-area values (R^sub 0^A) at 77 K of 4 × 10^sup 6^ ohmcm^sup 2^ at cutoff wavelength λ^sub CO^ - 4.5 µm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball bonding to the 1-D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The best measured noise equivalent temperature difference (NETD) on a LWIR array was 8 mK, with a median of 14 mK for the 42 operable diodes. The best measured NETD on a MWIR array was 18 mK. Two-D arrays showed reasonably good uniformity of R^sub 0^A and zero-bias current (I^sub 0^) values. The first 64 × 64 element 2-D array of 16 × 16 µm^sup 2^ MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK.

Key words: CdHgTe, HgCdTe, CdZnTe, long-wavelength infrared (LWIR), mid-wavelength infrared (MWIR), ion milling, molecular beam epitaxy, n-on-p diodes, photodiodes, planar diodes, linear array, two-dimensional (2-D) array, IR detector

INTRODUCTION

There are many methods for making pn junctions in Cd^sub x^Hg^sub 1-x^Te (CMT). These include ion implantation, in-situ growth of the dopants, and dry etching. Although fabricating high-quality diodes by any of these techniques is challenging, dry etching such as ion milling or reactive ion etching (RIE) is a relatively simple technique that has been studied in more detail recently.1

Ion milling of p-type vacancy-doped CMT releases Hg atoms that diffuse into the sample and fill in Hg vacancies, thereby revealing a background n-type carrier concentration and creating a pn junction.1-6 Experiments show that in thin layers (

Diodes can be fabricated in a planar process, in which the pn junction at the surface of the CMT is created under the passivation layer and is therefore never exposed to air. We have reported linear arrays of high-quality diodes fabricated in such a process earlier.8 In this paper, we present results from our first imaging one-dimensional (1-D) and two-dimensional (2-D) arrays, and we show that ion milling can be used to make arrays with good uniformity, high dynamic resistance and spectral response values, and low noise equivalent temperature difference (NETD) values.

The RIE on CMT also converts p-type material, with either vacancy- or extrinsic doping, to n-type, and small mid-wavelength infrared (MWIR) 2-D arrays with good device characteristics and longwavelength infrared (LWIR) test diodes have been reported recently.9-11

EXPERIMENT

The 9-10 m thick CMT layers with composition x ~ 0.2 and x ~ 0.3 were grown by molecular beam epitaxy (MBE) on (211)B oriented CdZnTe (CZT) substrates at a substrate temperature of ~195°C.12 The background n-type doping carrier concentration was 3-5 7times; 10^sup 15^ cm^sup -3^. In most of the layers, the Cd content x was varied by 0.02 over the thickness of the layer, with decreasing x-value toward the top of the layer. The gradient profiles are described in more detail in Ref. 8. The gradient creates a pseudoelectric field, which should help the electrons to move toward the pn junction and thereby be collected more efficiently, and previous results indicate higher and more uniform RA values on such layers. The 0.5 µm thick CdTe (CT) passivation layers were grown in situ at a substrate temperature of 250°C. The passivation layer is in reality a high x-value CMT layer, as there was still a Hg atmosphere in the growth chamber during this growth. The passivated layers were annealed in vacuum at 250°C for 24 h to give p-type vacancy doping with carrier concentrations at 77 K in the range (N^sub a^ - N^sub d^)^sub 77^ ~ 1 - 4.5 × 10^sub 16^ cm^sup -3^ and hole mobility -300 cm^sup 2^/Vs for MWIR and -600 cm^sup 2^/Vs for LWIR layers.

The 0.3 µm ZnS was then thermally evaporated onto some samples selected for 1-D arrays. ZnS is a much better insulator than CdTe, and this is important for the 1-D arrays, which have long lead-out wires and bonding pads deposited on top of the passivation. Large lead-out Au areas directly on top of the CT passivation were in some cases clearly seen to increase the leakage current and thereby decrease the R^sub 0^A values of the diodes. The effect of ball bonding was much worse, and often resulted in small or significant reduction in R^sub 0^A up to the point where a diode must be defined as inoperable. ZnS was deposited either on top of a photoresist mask which covered the diode areas, or on a larger rectangular mask which covered all the diodes and the area immediately surrounding them. In this way, we could use our regular method for diode processing (without etching ZnS), but have extra insulation under the bonding pads and most of the lead-out wires.