2. Point Defects. R. Krause-Rehberg

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1 R. Krause-Rehberg 2. Point Defects (F-center in NaCl) 2.1 Introduction 2.2 Classification 2.3 Notation 2.4 Examples 2.5 Peculiarities in Semiconductors 2.6 Determination of Structure and Concentration 2.7 Vacancies in thermodynamic Equilibrium 2.8 Irradiation-induced Point Defects 2.9 Aspects of Defect Chemistry

2 EL2 in GaAs: important antisite defect probably the frequently studied point defect at all: EL2 in GaAs used to obtain semi-insulating GaAs (auto-compensation of unwanted impurities) self-compensation works only when [EL2] > [shallow acceptors] > [shallow donors] step 3 needs too high temperature, thus all carriers are compensated at normal temperatures condition can be fulfilled in pure semi-insulating GaAs by doping with C RKR 2001 Structure of imperfect solids - Point defects in semiconductors 2

3 EL2 in GaAs: important Antisite Defect interesting feature: EL2 exhibits metastability illumination at low temperature properties changes (e.g. no IR absorption any more) many structural models were discussed Dabrowski/Scheffler and Chadi/Chang: EL2 is isolated As Ga and in metastable state the antisite atom moves outward and leaves a V Ga Metastability is lost during warming-up to 115 K RKR 2001 Structure of imperfect solids - Point defects in semiconductors 3

4 EL2 in GaAs: important Antisite Defect positron annihilation is a method to detect vacancy-type defects in solids before annihilation, diffusing positrons can be trapped by such defects as a consequence: positron lifetime increases due to the reduced electron density in the vacancy experiment shows the existence of a Ga vacancy in the metastable state of GaAs, which does not exist in stable ground state was prove of As Ga model of EL2 R. Krause, K. Saarinen, P. Hautojärvi, A. Polity, G. Gärtner, and C. Corbel Observation of a monovacancy in the metastable state of the EL2 defect in GaAs by positron annihilation Phys. Rev. Lett. 65 (26), (1990). RKR 2001 Structure of imperfect solids - Point defects in semiconductors 4

5 DX Center in GaAlSb defect appears in doped quasi-ternary III-V compound semiconductors (e.g. Al x Ga 1-x As, Al x Ga 1-x Sb) 10 2 GaAlSb:Te is complex: donor-? (so-called DX center) also shows metastable state at low temperatures model of Dabrowski/Scheffler predicted vacancy in stable state and the disappearance of this vacancy in metastable state Photoconductivity [S] Illumination proved by positron annihilation Ga Sb Te Al Ga Sb Te Al Average positron lifetime [ps] Illumination Annealing temperature [K] metastable stable R. Krause-Rehberg et al., Phys. Rev. B 48 (1993) RKR 2001 Structure of imperfect solids - Point defects in semiconductors 5

6 Compensating Defects in GaAs:Te Te is typical donor in GaAs is built-in only as Ta As experimental finding: with increasing donor doping concentration acceptor density simultaneously increases self-compensation degree of compensation about 25% confirmed model: donor acceptor Te As + V Ga Te As - driving force for generation of defect clusters: so-called Fermi-level effect it is energetically favorable to form additional acceptors in n-type GaAs RKR 2001 Structure of imperfect solids - Point defects in semiconductors 6

7 Compensating Defects in GaAs:Si Si is also often used as donor in GaAs Si is built-in as Si + Ga and also as SiAs - (amphoteric behavior) situation is different from GaAs:Te degree of compensation not constant, but growing result: doping only possible up to cm -3 at higher Si content: almost complete autocompensation model for additional compensating center (acceptor): V Ga Si - Ga RKR 2001 Structure of imperfect solids - Point defects in semiconductors 7

8 Compensating Defects in GaAs:Si model was proven by correlated STM and positron experiments STM shows at cleavage planes of GaAs:Si the V Ga Si - Ga defect (but possibly formed during cleavage) positron annihilation found the same number of vacancies in the volume of the identical crystals conclusion: both methods detect the identical defects J. Gebauer et al. Phys. Rev. Lett. 78 (1997) 3334

9 2.5 Peculiarities in Semiconductors defects in semiconductors can be charged (+, 0, -) charge depends on position of Fermi level electronic configuration and structure of a defect depend on charge state a different charge leads to different lattice distortions is so-called Jahn-Teller Effect thus: distortion energy depends on charge state influence may be so strong that normal charge sequence (2-, -, 0, +, 2+) is changed: negative-u behavior electron configuration of V 0 in Si RKR 2001 Structure of imperfect solids - Point defects in semiconductors 9

10 negative-u behavior example: theoretical calculations of ionization levels No. a) to c) are calculated without lattice relaxation calculations g) to h): lattice distortion was taken into account Jahn-Teller Effect is frequently found in semiconductors Ga As in GaAs Defects in InP

11 2.6 Determination of structure and concentration range of concentration: about cm -3 (metallic impurities in Si) > cm -3 (some dopants in Si, As Ga in LT-GaAs) defect identification difficult due to large variety in GaAs: 6 intrinsic defects in many charge states; they form defect complexes; in addition: they can form complexes with impurities there is no universal method (see lecture in next course) many methods give information about ionization levels in band gap, but no structural information (e.g. DLTS, Hall, IR absorption) other methods have structural information, but can be applied to only a restricted number of defects (e.g. EPR, Positron Annihilation) RKR 2001 Structure of imperfect solids - Point defects in semiconductors 11

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