Секция 2. Радиационные эффекты в твердом теле Section 2. Radiation effects in solids

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INVESTIGATION OF {111} HYDROGEN PLATELET DEFECTS IN PROTON BOMBARDED GaAs USING Cs-CORRECTED STEM

Johannes Neethling, Jaco Olivier, Arno Janse van Vuuren

Centre for HRTEM, Physics Department, Nelson Mandela University, Port Elizabeth, 6001, South Africa Jan.Neethling@mandela.ac.za, jaco.olivier@mandela.ac.za, arno.jansevanvuuren@mandela.ac.za

Probe-Cs-corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is used to image

loop-like defects on {111} planes in high-dose proton bombarded (1017 H+ cm-2) n-type GaAs. The typical {111} loop-like defect present at the peak of the H+ projected range in the 300 keV proton bombarded GaAs was found to consist of a single {111} GaAs layer with adjacent areas of lower crystal density which are suggested to be small hydrogen bubbles formed by H2 molecules trapped at tetrahedral sites adjacent to the {111} plane of the interstitial loop. It has been suggested that in GaAs, H diffuses in atomic form until trapped at a defect with the subsequent formation of a H2 molecule which is not mobile. This paper presents the first atomic resolution images of hydrogen platelet defects on {111} planes of high dose proton bombarded GaAs.

Keywords: GaAs; proton bombardment; hydrogen platelets; Cs-corrected scanning transmission electron microscopy.

Introduction Proton bombardment of GaAs using doses in the

range 1014 - 1015 H+ cm-2, has been used to create high-resistivity regions for GaAs-based device technologies [1]. Each implanted hydrogen (H) ion (proton) removes many electrons from the conduction band in n-type GaAs by trapping them at defects created by the implanted ions. The resistivity of the H ion implanted n-type GaAs returns to its pre-implanted value upon annealing around 500 °C. The remnant damage consists mainly of dislocation loops which appear to have only a minor effect on the electrical properties of the implanted layer [1]. Furthermore, hydrogen is an important impurity in GaAs and other semiconductors and has therefore attracted considerable attention over the years [2].

Although the first author of this paper (JN) has investigated radiation damage and the formation and growth of dislocation loops in n-type GaAs since the 1980s [3, 4, 5], the earlier transmission electron microscopy (TEM) studies of radiation damage in proton bombarded GaAs [3, 4, 5] were carried out long before probe-Cs-corrected HAADF STEM imaging became available and hence it was challenging to distinguish between an interstitial dislocation loop and a hydrogen platelet without being able to resolve the atomic structure of the loop or platelet directly.

Neethling and Phillip also compared the nature of loop-like defects on {111} planes, assumed to be hydrogen platelets, observed in proton bombarded (1017 H+ cm-2) n-type GaAs annealed at 523 K and in GaAs exposed to a hydrogen plasma at 453 K [6]. The <110> strain contrast bright-field TEM images of the platelets from the two samples appear to be similar and indicate a large stress component perpendicular to the {111} plane of the platelet. Both edge-on views of the {111} platelets show a line parallel to the {111} platelet plane exhibiting contrast typical of a two-dimensional array of bubbles (or voids). A HRTEM lattice image of the platelet in the proton bombarded sample (using a JEOL JEM-ARM 1250 high-voltage, atomic resolution microscope) revealed that the {111} planes adjacent to the platelet are displaced in a direction perpendicular to the platelet plane, assumed to be due the pressure generated by the hydrogen molecules [6]. However, HRTEM image characteristics are influenced by the foil thickness and defocus values

which make the interpretation of HRTEM images of lattice defects problematic.

The development of the Cs-corrected STEM with HAADF imaging mode has provided a powerful technique for determining structural and chemical information at atomic resolution since the image characteristics do not depend on foil thickness and defocus values. In earlier papers, the first author (JN) and co-workers concluded that the loop-like defects on {110} planes present in 300 keV proton bombarded (1015 to 1016 H+ cm-2) n-type Si-doped GaAs annealed in the range 500°C to 900 °C were hydrogen filled vacancy loops (called hydrogen platelets) [3, 4, 5] which, due to the gas pressure inside the loops, would appear to be of interstitial nature [3]. However, in a recent paper we compared earlier TEM results of dislocation loops generated in n-type GaAs by 1 MeV electron and fast neutron irradiation with recent Cs-corrected HAADF STEM imaging of {110} and {111} dislocation loops in neutron irradiated and annealed n-type GaAs [7].

The new data enabled us to reinterpret the older defect models [3, 4, 5] in proton bombarded n-type GaAs, i.e. the {110} loops in proton bombarded and annealed GaAs are now considered to be interstitial dislocation loops while hydrogen platelets form on {111} planes in high-dose proton bombarded GaAs (5 ×1016 – 1017) [7]. Cs-corrected HAADF STEM imaging of {110} and {111} dislocation loops in neutron irradiated and annealed (600°C for 20 minutes) n-type GaAs revealed that the plane of the {110} loop consists of two layers of GaAs atoms, which is consistent with the model for a {110} pure-edge interstitial dislocation loop in GaAs. The plane of the {111} interstitial loop consists of one layer of GaAs atoms while the stacking sequence across the loop is consistent with that of an extrinsic stacking fault [7]. Experimental

The n-type Si doped (1018 carriers cm-3) GaAs samples were bombarded with 300 keV protons at 7° off the [001] to total doses in the range 1015 to 1017 H+

cm-2. Post-implantation annealing was carried out in a tube furnace under flowing argon. HRSTEM specimens were prepared by using a Helios Nanolab 650 focused ion beam (FIB) SEM. HAADF STEM imaging was carried out using a double Cs-corrected JEOL ARM 200F operated at 200 kV.

Секция 2. Радиационные эффекты в твердом теле Section 2. Radiation effects in solids

13-я Международная конференция «Взаимодействие излучений с твердым телом», 30 сентября - 3 октября 2019 г., Минск, Беларусь 13th International Conference “Interaction of Radiation with Solids”, September 30 - October 3, 2019, Minsk, Belarus

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Results In a recent paper we concluded that dislocation

loops generated in n-type GaAs by 1 MeV electron and fast neutron irradiation annealed in the range 500 to 600 °C are interstitial loops on {110} and {111} planes [7]. Loops on {110} planes in proton bombarded n-type GaAs annealed in the range 500 – 800° C were also reinterpreted as interstitial [7]. In the current paper {111} loop-like defects in high-dose proton bombarded n-type GaAs were investigated using a Cs-corrected STEM to determine whether they could be hydrogen platelets as suggested earlier [6]. Fig. 1 shows a cross-sectional bright field TEM micrograph of the defect structure in n-type GaAs bombarded with 300 keV protons to a dose of 1017 cm-2. The arrow marked H+ indicates the direction of implantation. Loop-like defects on {111} planes at a depth of about 2.7 µm from the surface, are indicated by L. This depth coincides with the peak of the projected range of 300 keV protons in GaAs [5]. These loop-like defects were recently analysed using a Cs-corrected STEM and typical results are shown in Fig 2, where (a) and (b) are BF and HAADF STEM lattice images respectively of the {111} defects indicated by L in Fig.1.

Fig. 1. Cross-sectional bright field TEM micrograph of the defect structure in n-type GaAs bombarded with 300 keV protons to a dose of 1017 cm-2. The <001> direction of implantation is indicated by H+. Loop-like defects on {111} planes at a depth of about 2.7 µm from the surface, are indicated by L. The electron beam direction is <110>

In Fig. 2, the light (a) and dark (b) regions associated with the {111} plane of the defect viewed edge-on is indicative of regions in the GaAs with lower density, most likely due to the presence of hydrogen bubbles on the {111} plane of the defect. However, the higher magnification image of (b) shown in (c) also indicates that the defect consists of a single {111} layer of GaAs atoms.

Discussion

Loop-like defects on {111} planes at the projected

Fig. 2. Typical BF (a) and HAADF(b) STEM lattice images of the {111} loop-like defects indicated by L in Fig.1. (c) HAADF STEM lattice image of (b) at higher magnification. The <001> implantation direction is indicated in (a). The single {111} GaAs layer of the interstitial loop forming the core of the hydrogen platelet is indicated by the arrows in (b) and (c). The electron beam direction is <110>

H+ range in proton bombarded n-type GaAs were found to consist of a single {111} GaAs layer with adjacent areas of lower crystal density which are suggested to be small hydrogen bubbles formed by H2 molecules trapped at tetrahedral sites adjacent to the {111} plane of the interstitial loop. Theoretical

Секция 2. Радиационные эффекты в твердом теле Section 2. Radiation effects in solids

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calculations of hydrogen in pure and n-type GaAs indicated that the stable form of H is a H2 molecule in a tetrahedral site [8]. In GaAs, H atoms diffuse in atomic form until trapped at a defect where they react to form H2 molecules which are not mobile [8]. Conclusions

The typical {111} loop-like defect present at the peak of the H+ projected range in 300 keV high dose proton bombarded GaAs was found to consist of a single {111} GaAs layer with adjacent areas of lower crystal density which are suggested to be small hydrogen bubbles formed by H2 molecules trapped at tetrahedral sites adjacent to the {111} plane of the interstitial loop. These {111} interstitial loops with associated hydrogen bubble arrays (also referred to as hydrogen platelets [6]) observed in proton bombarded n-type GaAs (without post-bombardment annealing) (Fig. 2), differ from the {111} interstitial loops found in neutron irradiated n-type GaAs annealed at 600 °C [7]. The plane of the {111} interstitial loop in former case consist of one layer of GaAs atoms while the plane of the loop in the latter case consist of a single {111} layer of GaAs atoms with two rotated {111} layers forming an extrinsic stacking fault [7]. The positions of the Ga and As atoms (obtained by using EEL spectrum imaging) across the extrinsic stacking fault plane of the {111} loop indicate polarity continuation across the loop and the preservation of charge neutrality [7]. The difference between the two types of {111} interstitial loops observed in high dose proton bombarded (unannealed) and neutron irradiated (annealed at 600 °C) n-type GaAs is most likely due to firstly, the presence of H2 molecules at the loop plane in the proton bombarded samples which prevented the formation of an extrinsic stacking fault in the loop and secondly, the fact that the proton bombarded samples were not annealed after bombardment.

The development of the Cs-corrected STEM with HAADF imaging mode has provided a powerful technique for determining structural and chemical

information at atomic resolution. The nature of small dislocation loops in proton, electron and neutron irradiated n-type GaAs may be determined unambiguously by Cs-corrected HAADF STEM since the image characteristics do not depend on foil thickness and defocus values as is the case for HRTEM imaging. Acknowledgements

The National Research Foundation and Department of Science and Technology are acknowledged for financial support. References 1. Pearton S.J. Ion implantation for isolation of III-V

semiconductors. Mater. Sci. Rep. 1990; 4: 313-367. 2. Pearton S.J., Corbett J.W., Stavola M. Hydrogen in

Crystalline Semiconductors. Berlin: Springer-Verlag; 1992.

3. Neethling J.H., Snyman H.C. Identification of hydrogen platelets in proton-bombarded GaAs. J. Appl. Phys. 1986; 60(3): 941- 945.

4. Neethling J.H., Snyman H.C., Ball C.A.B. Analysis of transmission electron microscopy images of hydrogen platelets in proton-bombarded GaAs. J. Appl. Phys. 1988; 63(3): 704-711.

5. Neethling J.H. The effects of hydrogen dose and thermal treatment on the formation of microsplits in hydrogen implanted GaAs. Physica B 1991; 170: 285-291.

6. Neethling J.H., Phillipp F. The nature of hydrogen platelets in GaAs, Proceedings of the 14th International Congress on Electron Microscopy; 1998 Aug 31 – Sep 4; Cancun, Mexico; 1998, Vol. III, p. 413 – 414.

7. Neethling J.H., Janse van Vuuren A., Olivier E.J., van Aken P.A. TEM and HAADF STEM Imaging of Dislocation Loops in Irradiated GaAs. Acta Physica Polonica A 2019 (in press).

8. Pavesi L., Giannozzi P. Atomic and molecular hydrogen in gallium arsenide: A theoretical study. Phys. Rev. B 1992; 46: 4621-4629.