engleski

Wide band-gap II-VI compounds . can the efficient doping be achieved?

II-VI semiconductors, possessing the direct band-gaps from infrared to ultraviolet, have a potential for a variety of light-emitting devices spanning the entire range of the visible spectrum. The recent successes in obtaining first blue-green ZnSe-based lasers have greatly stimulated new experimental and theoretical investigations. However, persevering difficulties in achieving efficient doping still remain unsolved puzzle about this whole class of materials. Only CdTe can be easily doped both types, while ZnSe, CdSe and CdS can be easily doped only on n-side, and ZnTe (and newly sinthesized MgSe and MgTe) only on p-side. In this paper a number of possible doping-limiting mechanisms in II-VI´s and various approaches to avoid them or overcome are summarized and analyzed, including this author's contributions to these efforts. At the present state of knowledge the main mechanisms limiting efficient doping in equilibrium conditions can be grouped as follows: - Compensation by native defects and/or pairs: wide band gaps give a strong incentive for the introduction of compensating defects (native defects or fast contaminants), because compensation lowers the formation energy of these defects by an amount approximately equal to the band-gap energy for a singly ionized species, therefore effectively lowering the total energy of the crystal. This mechanism limits the n-type doping in CdTe and probably most other tellurides and selenides (with the exception of ZnTe and MgTe) although only at high donor concentrations, as well as p-type doping for some acceptor/compound combinations. - Large lattice relaxation around doping atom resulting in bonds breakage with host atom and formation of compensating deep localized levels. It seems that his mechanism strongly limits the p-type doping for V-column acceptors (except N) in most II-VI compounds and the n-type doping in ZnTe and MgTe. - For some dopants (Li, Na, Cu, Ag, also possibly N..) their amphoteric behavior (the tendency to occupy electrically opposite places in lattice, like substitutional-interstitial places) can cause auto-compensation and hence lower their efficiency as dopants, but it is not clear yet to which extent. - The insufficient solubility has been convincingly proven as a main culprit for some cases, like low p-doping in Na doped ZnSe, but it is suspected to be the contributing or even the main problem in many other cases as well. Solubility seems to be limited mostly by the formation of a second phase (formation of Li2Se, Na2Se, N3Se4, CdIn2S4 ...). The difficulty in evaluation of relative importance of particular mechanism arises primarily due to very similar energies of the formation/incorporation of the number of competing compensating defects or complexes, so that each combination of specific dopant/compound has to be studied separately. The magnitude of the problem is best illustrated with the (most studied) example of N doped ZnSe: N is an acceptor when placed at substitutional Se site, NSe but hole concentration cannot yet be obtained higher than ~(1-2)ˇ1018/cm3 even if N concentration, [N], exceeds 1019/cm3. Mechanisms suggested in various reports to explain this observed saturation of electrical activity included all of the above mechanisms (deactivation of N due to formation of the second phase N3Se4, compensation of acceptor NSe by native double donor- Se vacancy VSe, or donor pairs VSe - NSe or VSe - Ni, by interstitials Ni or Zni, by VZn - NSe antisite, by formation of N2 molecule, and by relaxation-related deep compensating levels). This author's contribution to the understanding and resolving doping problems includes particularly: a) Calculation of minimal degree of self-compensation by ionized native vacancy using two-band approach for practically all III-V and II-VI compounds (22 cases). The tendency toward self-compensation predicted from these calculations follow exactly the trends observed experimentally for all these compounds but in most cases this mechanism is insufficient to explain the extent of observed doping problems. b) From the study of Li as a dopant in CdTe it was found that at high-T substitutional LiCd (acceptor) is mostly compensated by interstitial LiI (donor). After quenching to RT, lower solubility of Li forces much more mobile LiI atoms to the surface/second phase leading to decompensation and strong p-type conductivity. c) Formulation of the method of "indirect doping" which proposes doping of desired dopant together with oppositely charged radioactive atoms. Example is donor 111In, which after decay to 111Cd (lifetime 2.8 days) becomes just an ordinary host atom in Cd compounds. However, while being a 111In donor it ensures the increased incorporation of desired co-doped acceptor and reduces the formation of other compensating (native) defects during the critical time of necessary thermal treatment. d) In a recent detailed study of In implanted CdS we have shown that ~100% implanted In atoms can be placed in a perfect, undisturbed, substitutional Cd place, InCd, for a very wide In dose range, 1016/cm3 -1019/cm3 in implanted layer. It was demonstrated, by comparison between macroscopic, electrical measurements and microscopic, Perturbed Angular Correlation measurements (which give information on immediate surrounding of 111In probe atom), that In donors in CdS can be either completely compensated by Cd vacancy, VCd, spontaneously created at next-neighbor place, or completely de-compensated (via removing of all Cd vacancies) by appropriate thermal treatments. For annealing conditions under S overpressure the concentration of thermally created VCd is proportional to [In] even up to [In]~1020/cm3. Renewed interest and extensive work done during last several years confirmed the seriousness of doping problem in II-VI compounds and revealed that almost every of potentially "good" dopants has one or several serious drawbacks. However, the greatly extended theoretical and practical understanding of the mechanisms causing doping problems in the same time showed the way toward their solution(s) and kept inspiring new approaches, like doping during low temperature crystal growth, doping by ion implantation, co-doping with more than one dopants, etc. The best way to solve the doping problems in II-VI compounds appears to be non-equilibrium processes, where excellent results have been obtained (particularly with MBE and other low-temperature growth/doping techniques) opening the ways for much wider applications of these materials in the near future.

II-VI compounds; defects; doping

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