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SPECIES-RICHNESS-IN-FLUCTUATING-ENVIRONMENTS - DOC

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SPECIES-RICHNESS-IN-FLUCTUATING-ENVIRONMENTS

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									P-TYPE ZNO:AS OBTAINED BY ION IMPLANTATION OF AS+ WITH
   POST-IMPLANTATION ANNEALING IN OXYGEN RADICALS


                        A. N. GEORGOBIANI AND V. I. DEMIN
                         P.N.Lebedev Physical Institute of RAS
                     53 Leninsky Prospekt, 119991 Moscow, Russia
                             E-mail: georg@sci.lebedev.ru

       M. B. KOTLYAREVSKY, I. V. ROGOZIN AND A. V. MARAKHOVSKY
                        Berdyansk State Pedagogical University
                     4 Shmidt Prospekt, 71100 Berdyansk, Ukraine
                              E-mail: rogozin@bdpu.org


     Zinc oxide is the promising material for creation of the new generation of
     detectors for particle physics and radiation dosimetry. It has been shown that
     ion implantation of arsenic into zinc oxide film (arsenic is an acceptor impurity
     in ZnO) can result in formation of the p–type conductivity only in case of
     annealing in the flux of atomic oxygen. The ion implantation and the following
     annealing had influenced not only electrical properties of ZnO:As + layers, but
     also their photoluminescence spectra. The ultraviolet luminescence band with
     the maximum at 3.33. eV corresponding to the As O acceptor center had been
     clearly observed in the spectra of ZnO films implanted by As + ions. The optimal
     temperature range of annealing in the atomic oxygen flux, required for
     obtaining of p–type conductivity in ZnO films, had been determined.


1.   Introduction

Zinc oxide is the promising semiconductor for creation of the new generation of
detectors for particle physics and radiation dosimetry. ZnO ,as well as diamond,
differs from silicon because of the much higher bandgap energy. Consequently,
the operating mode of the particle detectors based on them should be much
different and much easier than that for the silicon detectors. Also radiation and
chemical tolerance of ZnO is better than that of silicon. Creation of p-n junctions
is one of the important problems in producing semiconductor detectors based on
ZnO. The main difficulty in solving this problem is connected with the strong
tendency of wide-gap II-VI compounds towards monopolar n–type conductivity.
The introduction of acceptor impurities by means of traditional methods is
accompanied by the process of self-compensation because of the oxygen
vacancies generation, that act as donors in ZnO. The thermodynamic and kinetic
analysis made by us [1-3] has shown that it is possible to suppress the self-

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compensation in II-VI compounds only when the growth and doping temperature
in the atmosphere of the saturated vapour of non-metal component of the
compound don’t exceed the critical value. The expression determining the
critical temperature (Tcr) has been obtained. The calculation shows that Tcr for
ZnO is about 300 K. It is evident that at this temperature the processes of growth
and doping are “frozen”. The small value of Tcr is due to the fact that dissociation
energy of the oxygen molecule is rather high - about 5 eV, but only atomic
oxygen is active in the mentioned processes. The method called Radical Beam
Gettering Epitaxy (RBGE) has been elaborated on this basis [3-7]. The essence
of this method is the artificial dissociation of molecules in the atmosphere of
non-metal component, for example stimulated by radio frequency (RF) discharge.
This results in the growth of Tcr [6]. With the help of this method the p-type ZnO
samples had been obtained [4, 8, 9] In this case the intrinsic defects  zinc
vacancies – served as acceptors.
     The goal of this work is to investigate the potentiality of the RBGE method
in case of acceptor doping. Arsenic is one of the most promising acceptors for
ZnO . We have used the low-temperature method – ion implantation – for its
introduction.

2   Experimental methods
The initial ZnO films with thickness 0.1 m doped by gallium were obtained by
magnetron high frequency deposition on the amorphous SiO2 substrates at
temperature equal to 620 K. Ga2O3 was used as a source of gallium impurity.
According to the data obtained by Hall measurements, the layers possessed n-
type of conductivity. At room temperature the carrier concentration and mobility
were 2·1017 cm-3 and 120 cm2·V-1·s-1 respectively. The films were doped by As+
ion implantation with doses: 51014 ÷ 51015 cm-2. The energy of arsenic ions was
200 keV. Analysis of the impurity composition was performed by SIMS method.
    After implantation the films became amorphous, insulating, and non-
luminescent. Post-implantation annealing of radiation defects was necessary to
restore the crystal lattice and to control the composition of the point defects
aimed at the stoichiometry shift to oxygen surplus. Such an annealing was
realized with the help of RBGE method in the atmosphere of the atomic oxygen
during 30 minutes at different temperatures. Elemental oxygen was generated by
RF–discharge of 40 Wt power in oxygen atmosphere at pressure ~ 1 Pa. By
passing molecular oxygen through the RF-discharge region we obtained a
mixture of molecular and elemental oxygen and oxygen plasma. The charged
particles were removed by the applied magnetic-field, so that only oxygen
molecules and atoms reach the film surface. The concentration of atomic oxygen
                                                                                            3

was about 1017 cm-3. In case of thermal dissociation at 800 °C such a
concentration of atomic oxygen corresponded to oxygen pressure about 105 MPa,
a value which could not be attained because it is much greater than the saturated
vapour pressure at this temperature.
    The photoluminescence (PL) spectra of the films under excitation by the
pulse nitrogen laser at liquid helium temperature were measured. The spectra
were analyzed by means of the double grading monochromator. The film
thickness was measured by quartz thickness-meter during the deposition process.
The type of conductivity was determined by the sign of the thermal EMF signal.


3    Experimental results
After annealing of the implanted films with the help of the RBGE method they
became polycrystal, that allowed to conduct PL measurements. The investigation
of PL is one of the main stages necessary for the optimization of the treatment
regimes, of the conditions of introduction and activation of the acceptor
impurities.




Figure 1. PL spectra of the initial ZnO film (a) and of ZnO:As+ layers implanted with the
doses 5·1014 cm-2 (b) and 5·1015 cm-2 (c) after the annealing in oxygen radicals at 670 K
for 30 minutes.

     Figure 1 presents the influence of the arsenic ion implantation at PL spectra
of the ZnO films measured after post-implantation annealing at 670 K. In the
spectra of initial ZnO:Ga films the intense peak with the maximum at 3.36 eV,
and the green band at 2.4 eV are observed (curve a). The peak at 3.36 nm is
connected with the annihilation of excitons bound at neutral donors. The
estimation of energy location of the donor (Ed) according to the formulae (D0,X)
4

= Eg – Eex – 0.15Ed [10], where Eg = 3.43 eV, Eex = 60 meV, gives the value of
Ed ~ 65 meV. The peaks at 3.33 and 3.25 eV appear in the spectra of the samples
implanted with the dose 5·1014 cm-2 (curve b). Increase of the dose up to 5·1015
cm-2 leads to the significant growth of green emission (curve c).




Figure 2. PL spectra of ZnO:As+ layers implanted with the dose 1015 cm-2 after annealing
for 30 minutes in oxygen radicals flux at different temperatures: 670 K (a), 920 K (b) and
1170 K (c).

    The dependence of PL spectra of the layers implanted with the dose 1015 cm-2
on annealing temperature varying in the range 670 – 1170 K is shown in Fig 2.
In the range 670 – 970 K the annealing results in the growth of the intensity of
the 3.33 eV peak (curves a,b). The increasing of the annealing temperature up to
1170 K leads to the diminishing of its intensity.
     The SIMS profiles of arsenic distribution in the implanted layers in the
dependence on the annealing temperature are presented in Fig. 3. The rising of
the temperature results in the widening the profile (curve b). At the highest used
annealing temperature (1170 K) the considerable diffusion of the impurity into
the substrate bulk and the exhaustion of the surface region is observed (curve c).
    We have checked the conductivity type of the treated ZnO samples. The p–
type of conductivity was obtained in the samples annealed in oxygen radicals in
the temperature range 870 –970 K.
                                                                                        5




Figure 3. SIMS profiles of As distribution in ZnO film in the dependence on the annealing
temperature: 670K (a), 870 K (b) and 1170 K (c).



4   Discussion
According to the experimental results presented above, it is possible to attribute
the peak at 3.33 eV to the annihilation of the exciton bound at the acceptor
center – arsenic in oxygen sublattice – AsO. The estimation of energy location of
the acceptor according to the formulae (A0,X) = Eg – Eex – 0.08Ea, where Eex = 89
meV [11] gives the value of Ea ~ 140 meV. This result is in good concordance
with the data of [12], in which the depth of the acceptor level connected with
arsenic was estimated. The peak with the maximum at 3.25 eV could be
connected with the annihilation of excitons at the defect complexes (VZn – VO)+.
Such a supposition is done because the shifting of the stoichiometry of ZnO to
the side of oxygen surplus results in the diminishing of the number of its
vacancies. In this case, the electro-neutrality occurs due to re-charging of oxygen
vacancies to the positive double-charged state, and consequently, to the
formation of defect complexes of (VZn – VO)+ type [2].
     The peaks at 3.33 and 3.25 eV are present only in the arsenic implanted
samples. The intensity of 3.33 eV peak reaches its maximum value at the
annealing temperature 920 K (Fig. 2, curve b). The existence of exciton
luminescence testifies to the restoration of the crystal structure. It is likely that at
low annealing temperatures As ions are located not only in the oxygen sites. The
increasing of the annealing temperature leads to the effective immersion of As
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into the latter. The diminishing of the intensity of the mentioned peaks at the
annealing temperature 1170 K is evidently due to the evaporation of As from the
layers, that is proved by the data of SIMS analysis (Fig. 3).


Acknowledgments

This work was supported by RFBR – NSFCC grant No 02-02-39007, by the
Ministry of Industry, Science and Technology of Russian Federation – Programs
“Inequilibrium phenomena in semiconductor nanostructures”, “Elaboration of
compact sources of coherent emission” and by Russian Academy of Sciences. –
Programs “Physics of solid-state nanostructures”, “Semiconductor lasers”, “New
materials and structures”.


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