Deactivation of Arsenic as an Acceptor by Ion Implantation and Reactivation by Low-Temperature Anneal

Journal of Electronic Materials, Jun 2005 by Chandra, D, Schaake, H F, Kinch, M A, Dreiske, P D, Et al

Arsenic incorporated in HgCdTe epifilms at levels ranging from 6 � 10^sup 14^ to 4 � 10^sup 16^ cm^sup -3^, and activated completely as an acceptor, converts into a donor upon introduction of specific damage introduced by ion milling, ion implantation, or ECR. The results could be correlated to the formation of a complex between the arsenic and a product of the lattice damage process. If this product is assumed to be an atom, which could be an Hg interstitial, then a direct mass balance relationship dictates the formation of one Hg interstitial for each 49,000 atoms of Hg removed by the damage process. A very similar relationship is observed in the annihilation of metal vacancies pre-existing in the solid by damage introduced by ion milling. The deactivated arsenic, present as a donor, can be restored back to complete activation as an acceptor upon annealing for relatively short durations at 120�C.

Key words: HgCdTe, As deactivation, ion-implantation, annealing

INTRODUCTION

Arsenic can be introduced precisely and reproducibly into HgCdTe epitaxial films for concentrations at

Diode formation at DRS is by the introduction of a damage process. Arsenic doping is of relevance here as a means of producing lightly p-doped HgCdTe for use in forming high density vertically integrated photodiode (HDVIP) diodes. Figure 1 describes a unit cell of the HDVIP diodes. The objective is to form n^sup ^-p^sup -^ diodes, with the n^sup ^ region produced by electron cyclotron resonance (ECR) etching and ion implantation. When arsenic activated as an acceptor is employed, the diode formed appears to be a n^sup ^- n^sup -^-p^sup ^ structure with a wide n^sup -^ region. The diode forming damage process appears to convert the arsenic into a donor. This conversion has been studied during the present investigations. Furthermore, methods have been discovered during these investigations to convert this arsenic back to an acceptor at low temperatures.

EXPERIMENT

The following two types of experiments were conducted.

* To elucidate information on the conversion process from p to n, we want to determine whether (a) arsenic merely neutralizes partially or completely during the diode forming damage process, or (b) alternatively, whether an n-type defect is being formed.

* If the result of the above investigations leads to the conclusion that an n-type defect is being formed, then a second series of investigations would be conducted, where we want to determine if this n-type material can be converted back to p-type by processes, which would not damage the diode, formed. These processes may include low-temperature annealing.

A series of films were grown employing a vertical dipping infinite-melt Te-rich liquid-phase epitaxy. The arsenic incorporated in the first group of films was targeted at 6 � 10^sup 14^ cm^sup -3^, increased subsequently to 8.0 � 10^sup 14^ cm^sup -3^ for a second series of films, and increased further to 1.0 � 10^sup 15^ cm^sup -3^ for a third series of films grown. These individual levels were confirmed directly by SIMS measurements1 on selected specimens from each group. A series of films were also grown from a vertical dipping infinite-melt Hg-rich LPE. The arsenic incorporated in these films was targeted at 3.5 � 10^sup 16^ cm^sup -3^. Direct measurement by SIMS indicated the arsenic level of these films to be 3.6 � 10^sup 16^ cm^sup -3^.

All these films were subjected to an activation anneal followed by a stoichiometric adjustment anneal, as described previously. For good measure, the films grown from the Hg-rich LPE process were also subjected to this process despite the fact that the arsenic incorporated should already be completely activated as acceptors, occupying sites in the Te sublattice.

"Heavy" Ion Milling

This process was employed to attempt to convert the entire arsenic present within the "bulk" of the films. Two limiting cases were selected, one with the lowest level of arsenic incorporated (6 � 10^sup 14^ cm^sup -3^) and the other with the highest level of arsenic introduced (3.6 � 10^sup 16^ cm^sup -3^). During this process, 0.85 �m of HgCdTe was removed from the surface. Hall measurements were made before and after etching.

"Variable" Ion Milling

This process was employed to attempt to determine the depth to which arsenic converted when either the degree of damage or the level of arsenic were progressively varied. Differential Hall measurements were employed to determine the depth of conversion from p-type to n-type.

"Variable" Ion Milling of Vacancy-Doped MCT

This process was employed to compare the depths determined above with the depths of conversion determined in MCT films without any arsenic, but preannealed to deliberately introduce a predetermined magnitude of metal vacancy concentration.

RESULTS AND DISCUSSION

Conversion of Arsenic

The n-type carrier concentration after heavy ion milling (or "etch") appears to be equal to the p-type carrier concentration before milling. This is shown in Table I.

Hence, we must be forming an n-type defect during ion milling; arsenic is not merely being neutralized. To understand the conversion process, a number of films were subjected to relatively light ion milling to produce a p-n junction within the film. The junction depth was correlated with the amount of material removed during the milling and the carrier concentration of the arsenic in the film.