Ion-implanted, shallow-energy, donor centres in diamond: the effect of negative electron affinity.

1. Ion-implanted, shallow-energy, donor centres in diamond: the effect of negative electron affinity. Johan F. Prins Department of Physics, University of Pretoria, Pretoria 0002 Gauteng, South Africa. 4th International Conference on: Radiation Effects on Semiconductor Materials Detectors and Devices July 10-12, 2002 Florence, Italy

2. Questions of faith:

3. Questions of faith: • Do you believe that the Second Law of Thermodynamics • can be negated? Energy generation from nothing? Perpetual motion?

4. Questions of faith: • Do you believe that the Second Law of Thermodynamics • can be negated? Energy generation from nothing? Perpetual motion? • Do you believe that Heisenberg’s Uncertainty Relationship • is a fundamental law of Physics? Does God play dice?

5. Questions of faith: • Do you believe that the Second Law of Thermodynamics • can be negated? Energy generation from nothing? Perpetual motion? • Do you believe that Heisenberg’s Uncertainty Relationship • is a fundamental law of Physics? Does God play dice? 3. Do you believe that the Pauli Exclusion Principle is a fundamental law of Physics? Can the periodic table be explained without Pauli?

6. Questions of faith: • Do you believe that the Second Law of Thermodynamics • can be negated? Energy generation from nothing? Perpetual motion? • Do you believe that Heisenberg’s Uncertainty Relationship • is a fundamental law of Physics? Does God play dice? 3. Do you believe that the Pauli Exclusion Principle is a fundamental law of Physics? Can the periodic table be explained without Pauli? I believe in all three wholeheartedly!!

7. A simple circuit diagram I  0 V1  V2

8. A simple circuit diagram I  0 V1  V2 Current flows through S without a field in S

9. A simple circuit diagram I  0 V1  V2 Current flows through S without a field in S Resistance of S is zero

10. A simple circuit diagram I  0 V1  V2 Current flows through S without a field in S Resistance of S is zero S is superconducting!

11. Outline of talk: • 1. Introduction: • n-Type doping and cold cathode action from such diamond? • 2. Formation of shallow donor levels in diamond: • Ion implantation of O+- or N+-ions to form metastable flaws. • 3. Donor levels above the vacuum level: • (a) Conditions at interface to vacuum • (b) Extraction of electrons at the interface • 4. Electron tunnelling into vacuum. • 5. An experimental result. • 6. Superconduction is required for steady-state current flow. • 7. Electron pair formation without phonon interaction. • 8. Formation of a Bose-Einstein Condensate. • 9. Conclusion

12. Negative Electron Affinity (NEA) of (111) p-type diamond surface terminated with hydrogen atoms Himpsel et al. Phys. Rev. B 20, 624-627 (1979)

13. Expected behaviour of n-type diamond with negative electron affinity

14. The quest for n-type diamond Substitutional nitrogen donor at 1.7 eV below conduction band: Too deep for room temperature conduction Phosphorus-doped diamond now being generated by CVD-growth or ion implantation:  0.6 eV below conduction band: Better, but electron extraction could not be obtained to date.

15. The quest for n-type diamond Substitutional nitrogen donor at 1.7 eV below conduction band: Too deep for room temperature conduction Phosphorus-doped diamond now being generated by CVD-growth or ion implantation:  0.6 eV below conduction band: Better, but electron extraction could not be obtained to date. Is diamond inherently a negative electron affinity material?

16. Mechanism of electron affinity change Ristein, Maier, Riedel, Cui and Ley, Phys. Stat. Sol. (a) 181, 65-76 (2000)

17. What if diamond is inherently a negative electron affinity material?

18. What if diamond is inherently a negative electron affinity material? Shallow donor levels imply that they should be at energies higher than the vacuum level!

19. What if diamond is inherently a negative electron affinity material? Shallow donor levels imply that they should be at energies higher than the vacuum level! A shallow donor level, most probably, needs to be an anti-bonding electron orbital!

20. What if diamond is inherently a negative electron affinity material? Shallow donor levels imply that they should be at energies higher than the vacuum level! A shallow donor level, most probably, needs to be an anti-bonding electron orbital! Flaws in the lattice, at which such levels form, will thus, most probably, have to be metastable defects.

21. What if diamond is inherently a negative electron affinity material? Shallow donor levels imply that they should be at energies higher than the vacuum level! A shallow donor level, most probably, needs to be an anti-bonding electron orbital! Flaws in the lattice, at which such levels form, will thus, most probably, have to be metastable defects. This could be the reason why it has been difficult to dope diamond n-type with shallow donors.

22. “Quenching” in shallow donor levels: Ion implantation at liquid nitrogen temperature, followed by rapid heating to a relatively low temperature (500 oC), was used to ”quench” in suitable, shallow donor levels, which are believed to be metastable: O+-ions give an activation energy of  0.32 eV J F Prins Phys. Rev. B 61 (2000) 7191. N+-ions give an activation energy of  0.28 eV J F Prins Semicond. Sci. Technol. 16 (2001) L50.

23. “Quenching” in shallow donor levels: Ion implantation at liquid nitrogen temperature, followed by rapid heating to a relatively low temperature (500 oC), was used to ”quench” in suitable, shallow donor levels, which are believed to be metastable: O+-ions give an activation energy of  0.32 eV J F Prins Phys. Rev. B 61 (2000) 7191. N+-ions give an activation energy of  0.28 eV J F Prins Semicond. Sci. Technol. 16 (2001) L50. The lattice flaws responsible for these levels are believed to be the implanted atoms trapped next to vacancies and “chemically bonded” to them.

24. Well, then go ahead and extract electrons!!

25. Well, then go ahead and extract electrons!! However, this diagram is not physically possible! A dipole has to form at the interface to the vacuum to establish Thermodynamic equilibrium.

26. Equilibrium dipole at the surface of an n-type semiconductor with negative electron affinity

27. Attempting to extract electrons from an n-type semiconductor with negative electron affinity

28. Conditions required to keep on extracting electrons from the n-type semiconductor, with NEA, into the electron-charge layer:

29. Conditions required to keep on extracting electrons from the n-type semiconductor, with NEA, into the electron-charge layer: It is essential to over-dope the near-surface region with shallow energy donors in order to facilitate tunnelling of the electrons from the conduction band through the surface

30. When does current flow between the semiconductor and anode initiate?

31. When does current flow between the semiconductor and anode initiate? When, at a critical applied potential C, the extracted electrons fill the whole gap between them: Electrical contact is then established between the semiconductor and the anode.

32. When does current flow between the semiconductor and anode initiate? When, at a critical applied potential C, the electron-charge layer fills the whole gap between them: Electrical contact is then established between the semiconductor and the anode. The applied potential now manifests itself as an offset between the Fermi levels of the semiconductor and the anode

33. Critical potential C at which current flow into anode initiates:

34. Acceleration of classical electrons from cathode to anode: z cathode anode

35. Schematic of equipment used to extract electrons from oxygen-ion doped diamond

36. An experimental result: electron extraction from oxygen-doped diamond:

37. Did we miss something?

38. The field in the gap does more than only accelerate electrons!

39. The field in the gap does more than only accelerate electrons! As long as there is a field within the gap, at the diamond’s surface, the depletion layer below the surface will increase in width, and thus CONTINUE TO INJECT ELECTRONS INTO THE GAP!

40. The field in the gap does more than only accelerate electrons! As long as there is a field within the gap, at the diamond’s surface, the depletion layer below the surface will increase in width, and thus CONTINUE TO INJECT ELECTRONS INTO THE GAP! The electron density in the gap keeps on increasing

41. Current flow through diamond and gap into the anode gap: electron density ngap anode diamond diam gap Janode Jdiam Applied potential:  = diam + gap Through diamond: Jdiam = (diam/ddiam) Into anode: Janode = engap(2e gap/m)1/2 Current: I = AdiamJdiam = AanodeJanode

42. Current flow through diamond and gap into the anode gap: electron density ngap anode diamond diam gap Janode Jdiam Applied potential:  = diam + gap Through diamond: Jdiam = (diam/ddiam) Into anode: Janode = engap(2e gap/m)1/2 Current: I = AdiamJdiam = AanodeJanode The current I keeps on increasing while Egap = gap/dgap keeps on decreasing, for as long as Egap  0

43. An unequivocal conclusion:

44. An unequivocal conclusion: Steady-state current flow, as required by the Second law of Thermodynamics, can only be achieved when Egap = 0

45. An unequivocal conclusion: Steady-state current flow, as required by the Second law of Thermodynamics, can only be achieved when Egap = 0 For Egap to become zero, the electrons within the gapHAVE TO FORM A SUPERCONDUCTING PHASE!

46. An unequivocal conclusion: Steady-state current flow, as required by the Second law of Thermodynamics, can only be achieved when Egap = 0 For Egap to become zero, the electrons within the gapHAVE TO FORM A SUPERCONDUCTING PHASE! If this has to happen, Mother Nature will find a way!!!

47. The conditions which (we now know) are required to form a superconducting phase:

48. The conditions which (we now know) are required to form a superconducting phase: • The charge carriers must have integral spin: i.e. • they must act like bosons.

49. The conditions which (we now know) are required to form a superconducting phase: • The charge carriers must have integral spin: i.e. • they must act like bosons. • The charge carriers must form a single, collective • and coherent wave function similar to a: • Bose-Einstein Condensate.

50. Increasing electron density and the Heisenberg Uncertainty Relationship