1. Alexandr V. Gradoboev, Doctor of Technical Science, professor
Influence of preliminary irradiation by gamma-quanta on development of catastrophic failures during operation of IR-LEDs
Alexandr V. Gradoboev, Doctor of Technical Science, professor
Tomsk – 2016

2. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Urgency
Infrared wavelength range light-emitting diodes (IR-LEDs) are used in various microelectronic devices operated in space conditions and at nuclear power plants. Operation conditions of LEDs require knowledge of their durability and reliability with complex and combined influence of radiation resistance and long operating time factors[1]. These show the urgency of the complex ensure of their radiation resistance and reliability.
[1] A.V. Gradoboev and A.P. Surzhikov, The Radiation Resistance Microwave Devices Based on Gallium Arsenide (Tomsk Polytechnic University, Tomsk, 2005), p. 277.
Alexandr V. Gradoboev, ,

3. Purpose of the research
– research the influence of preliminary irradiation by 60Co gamma-quanta on catastrophic failure (CF) development in the IR-LEDs during long- term operation. So, we have investigated combined influence of gamma-quanta and long-term operation factors on probability of CF development.
Figure 1 IR-LEDs based upon dual AlGaAs heterostructures
Alexandr V. Gradoboev, ,

4. Objects of the research
– industrial LEDs manufactured on the basis of dual AlGaAs heterostructures with ~1 μm active layer grown on the monocrystalline n+-GaAs wafer by means of liquid epitaxy.
Figure 2 The thickness of the layers of AlGaAs double heterostructures, microns
Alexandr V. Gradoboev, ,

5. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Research methods
Operating parameters of LEDs in continuous power mode:
Parameters of step-by-step tests:
Number of LEDs in each batch –
Forward operating current
20 items,
Iop = 50 mA;
ambient temperature T0 = 65ºС;
supply voltage Uop < 2 V;
increment step of current
maximum radiant wavelength
Istep = 50 mA;
 = 0.82 – 0.9 μm.
operating current of the first step
I1 = 50 mA;
duration of each stage t = 24 h.
Alexandr V. Gradoboev, ,

6. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Step-by-step tests
Active layer temperature of LED crystal:
RT – thermal resistance.
During step-by-step tests two types of failure were identified:
Catastrophic failure:
А-А – mechanical destruction plane at the T1;
В-В – crystal destruction plane at the T2, besides  T1 <  T2;
Parametric failure.
Figure 3 Standard structure of LED: 1 - crystal 2 - the lens and optical compound package, 3 - power lead.
Alexandr V. Gradoboev, ,

7. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Research methods
LEDs
Second batch
(LED-2)
First batch (LED-1)
Third batch (LED-3)
LED-1 – step-by-step tests without preliminary irradiation;
LED-2 – with preliminary irradiation with dose equaled to Dγ1 = 5∙104 Gy;
LED-3 – with preliminary irradiation with dose equaled to Dγ2 = 2∙106 Gy.
Preliminary irradiation was led to 60Co gamma-quanta without the adding external electric field.
Alexandr V. Gradoboev, ,

8. Rationale for choosing the doses of preliminary irradiation
5 ∙ 104
2 ∙ 106
Figure 4 Change of emissive power for LED-1 [1] (left) without preliminary irradiation batch depending on step number at operating current equaled 50 mA and LED-12 and LED-2 [2] (right) depending on irradiation dose by gamma-quanta: 1 – first stage of emissive power fall, 2 – second stage of emissive power drop.
[1] Gradoboev A.V., Simonova A.V., Orlova K.N. Physica Status Solidi C: Current Topics in Solid State Physics. In Press.
[2] Gradoboev A.V., Sednev V.V. Izvestiya vyzov. Fizika. – 2014, т.57, № 10/3. – Р. 20.
Alexandr V. Gradoboev, ,

9. Measurement of LEDs parameters
volt-ampere (V-I) characteristics;
watt-ampere (W-I) ball characteristics.
Equipment «Poli-500»:
forward voltage: 0 – 5 V;
forward current: 1 – 500 mA;
step: at least 1 mA;
error of forward current setting from given level – ± 2%;
error of forward voltage measurement – ± 2%;
error of emissive power measurement of LEDs doesn’t exceed 5%.
Figure 5 Equipment for measurement of LEDs parameters “Poli-500”.
Alexandr V. Gradoboev, ,

10. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Equipment «Explorer»
Isotopic continuous source of gamma radiation;
emitter isotope – 60Со;
dose rate ~ 1 Gy/s;
average gamma-quanta energy – 1,25 MeV;
certified volume – 3 litres;
irradiation time absorbed radiation dose.
Figure 6 Gamma-installation «Explorer»
Alexandr V. Gradoboev, ,

11. Initial characteristics of LEDs
Figure 7 Description of V-I characteristic of initial LEDs: 1 – the measured V-I characteristic, 2 – model description of V-I characteristic before connection of dislocations, 3 – total V-I characteristic of active layer of LED and dislocations, 4 – demonstration of ohmic contacts resistance in the high current area.
Figure 8 Equivalent circuit of LED: Rc1, Rc2 – ohmic contacts resistance, p-n-LED – p-n-junction (active layer) of LED, p-n-Disl –dislocation with derived connection to p-n-junction of LED, RDisl – resistance of dislocations.
Alexandr V. Gradoboev, ,

12. V-I characteristics of LED-1 batch
CF probability of LEDs during step-by-step tests is determined by accelerated degradation of ohmic contacts and active layer temperature of LEDs.
Figure 9 V-I characteristics for the LED-1 batch during step-by-step tests: A – initial; B – during step-by-step tests; C – before the CF development.
Alexandr V. Gradoboev, ,

13. V-I characteristics of LED-2 batch
The CF development is determined only by temperature of active layer of LED.
Preliminary irradiation by gamma-quanta with the selected dose make it possible to minimize additional contribution of ohmic contacts to CF probability during step-by-step tests.
Figure 10 V-I characteristics for the LED-2 batch during step-by-step tests: A – initial; B – during step-by-step tests; C – before the CF development.
Alexandr V. Gradoboev, ,

14. V-I characteristics of LED-3 batch
The selected dose of the preliminary irradiation by gamma-quanta leads to accelerated degradation of ohmic contacts.
The CF develops on the earlier stage of the tests.
Figure 11 V-I characteristics for the LED-3 batch during step-by-step tests: A – initial; B – during step-by-step tests; C – before the CF development.
Alexandr V. Gradoboev, ,

15. Influence of preliminary irradiation on CF probability
Probability of CF development during step-by-step tests is defined by the increase of ohmic contacts resistance and the temperature of active layer of LED.
Figure 12 The influence of different doses of preliminary irradiation by gamma-quanta on CF development depending on step number: 1 – LED-1; 2 – LED-2; 3 – LED-3.
Alexandr V. Gradoboev, ,

16. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Conclusion
Investigated LEDs had s-shaped V-I characteristics because of derived connection of dislocations to active layer of LED.
In the result of investigation we have established that the limit step of tests is determined by catastrophic failures due to mechanical destruction of LED package.
The significant changes of V-I characteristics are distinctive under influence both ionizing radiation and long-term operation factors. Observable changes of V-I characteristics are indicative of accelerated ohmic contacts degradation of LEDs.
CF probability of LEDs during step-by-step tests is determined accelerated degradation of ohmic contacts and temperature of active layer of LEDs.
Alexandr V. Gradoboev, ,

17. Alexandr V. Gradoboev, +79138668405, gradoboev1@mail.ru
Conclusion
5. Preliminary irradiation by gamma-quanta with doses corresponded to the radiation- stimulated reconstruction of the initial defect structure of the LED crystal improves ohmic contacts resistance to influence of long-term operation factors. Thus, it leads to decrease the CF probability at the early stage of step-by-step tests and rise the reliability of LEDs. 6. Preliminary irradiation by gamma-quanta with doses corresponded solely to the introduction of radiation defects leads to ohmic contacts degradation and increase of CF probability at the early stage of step-by-step tests. Consequently, the reliability of LEDs drops. Preliminary irradiation by gamma-quanta can be used in the manufacturing technology of LEDs for improvement their operational characteristics provided irradiation with doses in the strictly regulated field of radiation-stimulated reconstruction of the initial defect structure of the LED crystal.
Alexandr V. Gradoboev, ,

18. Thank you for attention
Alexandr V. Gradoboev, ,