Shyam M. Khanna

2 MeV proton radiation damage studies of gallium nitride films through low temperature photoluminescence spectroscopy measurements

Gallium nitride (GaN) thin film samples were grown by ammonia-molecular beam epitaxy. Through room temperature transport measurements, electron mobilities of 560 cm/sup 2//Vs were observed for layers with a carrier density of 1.5/spl times/10/sup 17/ cm/sup -3/. Room temperature photoluminescence (PL) spectroscopy revealed the bound exciton transition at 363.0 nm and a weak yellow emission whose intensity was sample dependent. At 22 K, the main photoluminescence signal sharpened, shifted to 356.9 nm (3.474 eV), and the maximum intensity increased by a factor of one hundred; the intensity of the yellow emission decreased. The samples were irradiated at room temperature with 2 MeV protons at fluences of 10/sup 9/, 10/sup 10/, 10/sup 11/, 10/sup 12/, 10/sup 13/, 10/sup 14/, 10/sup 15/, and 10/sup 16/ cm/sup -2/. The intensity changes were within experimental error up to 10/sup 13/ cm/sup -2/. The drop in intensity of the bound exciton transition was 16% at 10/sup 14/ cm/sup -2/ and 99% at 10/sup 15/ cm/sup -2/. The radiation damage constant associated with the main PL peak at 3.474 eV in GaN is (1.4/spl plusmn/0.3)/spl times/10/sup -13/ cm/sup 2/, compared with (4/spl plusmn/1)/spl times/10/sup -11/ cm/sup 2/ associated with the main PL, peak at 1.492 eV in GaAs. For photonic applications, GaN is more robust than GaAs with respect to displacement damage.

High-energy proton irradiation effects in GaAs devices

In this paper, we compare the energy dependences (53 and 115 MeV) of proton displacement damage coefficients for p/sup +/n GaAs solar cells with previously reported calculations of nonionizing energy loss (NIEL). Deep level transient spectroscopy (DLTS) was used to generate damage coefficients from the introduction rates of defects. New damage coefficients generated from GaAs bulk LEDs light output (1-530 MeV) are also reported. The damage coefficients from these devices for proton energies E>10 MeV vary but are bounded by the total and Coulombic NIEL.

Transport properties of proton-irradiated gallium nitride-based two-dimensional electron-gas system

A two-dimensional electron gas system (2DEG) is formed at the interface when a ternary alloy of aluminum gallium nitride is grown on gallium nitride. Very high carrier density can be achieved in these systems due to the strong piezoelectric and pyroelectric properties of the nitrides. The device was grown by molecular beam epitaxy and magnetron sputtering epitaxy. Through resistivity and Hall effect measurements in the temperature range 2 K to 300 K, its 2DEG conductive character, high mobility, and carrier density properties were confirmed. The effects of 2-MeV proton radiation on these properties are reported for the fluence range 1 /spl times/ 10/sup 13/ to 7 /spl times/ 10/sup 15/ cm/sup -2/. As a result of irradiation, the carrier density decreases by a factor of two whereas the mobility degrades by about a factor of a thousand. A fluence between 3 /spl times/ 10/sup 14/ cm/sup -2/ and 3 /spl times/ 10/sup 15/ cm/sup -2/ is necessary to drive a conductor to insulator transition for this 2-D gas and the change of phase is attributed mainly to changes in the mobility. This change of phase is determined by quantum conditions and could be used to establish an absolute standard for radiation damage.

Electron and neutron radiation-induced order effect in gallium arsenide

Electron (7 MeV) and neutron (1 MeV equivalent fluence damage in silicon) radiation effects in GaAs grown by the metallorganic chemical vapor deposition method are investigated. One series of samples was intentionally undoped, and another was doped n-type to 2.5*10/sup 15/ Si/cm/sup 3/. The fluences ranged from 10/sup 10/ to 6*10/sup 15/ cm/sup -2/ for electron irradiation and from 10/sup 12/ to 3*10/sup 15/ cm/sup -2/ for fission spectrum neutron irradiation expressed as 1 MeV equivalent fluence in silicon. The radiation damage was characterized by low-temperature photoluminescence (PL) measurements using 1.58 eV laser excitation, deep level transient spectroscopy and transport measurements. The observed decrease of trap concentration accompanied with an increase in PL intensity at lower fluences, an increase in the density of traps at higher fluences, and a fluence-dependent oscillatory PL intensity for acceptor levels indicate radiation-induced order at low fluences following by nonuniform reorganization of defects with radiation in GaAs. >

Spectral properties of proton irradiated gallium nitride blue diodes

The permanent damage induced by 2 MeV proton irradiation at room temperature is reported for gallium nitride based blue emitting diodes (CREE model C430-DH85). Both optical and electrical device characteristics were measured. The I-V dependence was obtained as a function of temperature. At low voltages, the current is proportional to the exponential of the voltage at a constant temperature and the slope of the I-V curve is independent of temperature for the range 75-350 K, confirming the tunneling mechanism of the carrier injection. The room-temperature curve was studied as a function of 2-MeV proton irradiation in the fluence range 10/sup 11/ to 10/sup 15/ cm/sup -2/. It is hardly affected up to a fluence of 3/spl times/10/sup 12/ cm/sup -2/. Higher fluences do not affect the tunneling mechanism, but proton irradiation affects the saturation value of the current. The integrated electroluminescence versus voltage curves were obtained as a function of fluence, but the results were not amenable to a degradation constant interpretation. To gain insight into the degradation mechanism, the electroluminescence was analyzed spectrally and found to be the sum of the band-to-band transition in blue color at /spl ap/430 nm and a parasitic yellow band. The contribution of each transition was determined. The ratio of the contributions depends on driving current, temperature, and fluence. Treated individually, both the band-to-band and the yellow transition are related to fluence. The 2-MeV proton radiation damage constant is (7/spl plusmn/1)/spl times/10/sup -14/ cm/sup -2/ for the band-to-band and (2.0/spl plusmn/0.4)/spl times/10/sup -14/ cm/sup -2/ for the yellow transitions. The degradation of space charge recombination and diffusion of minority carriers cause the degradation of the electroluminescence. GaN light-emitting diodes (LEDs) are about two orders of magnitude more resistant to 2-MeV proton irradiation than GaAs LEDs.

Proton energy dependence of the light output in gallium nitride light-emitting diodes

Gallium nitride (GaN)-based blue-emitting diodes (CREE Model C430-DH85) were irradiated at room temperature with protons in the energy range 2 to 115 MeV at fluences varying from 1/spl times/10/sup 11/ to 1/spl times/10/sup 15/ cm/sup -2/. Light output degradation curves were obtained for each energy and the damage constant (A) associated with these curves was determined according to the theory of Rose and Barnes. For proton energies less than 10 MeV, A varies inversely with the proton energy (E). At higher energies, A is consistently above the 1/E relationship. The change in nature of the energy dependence is attributed to nuclear interactions. Nonionizing energy loss calculations for the case of protons on GaN are presented. Good agreement between theory and experiment is obtained.

A mobility study of the radiation induced order effect in gallium arsenide

N-type gallium arsenide doped with silicon was irradiated with reactor neutrons to 10/sup 12/, 3/spl times/10/sup 12/, 10/sup 13/, 3/spl times/10/sup 13/, 10/sup 14/, 3/spl times/10/sup 14/, 10/sup 15/, and 3/spl times/10/sup 15/ cm/sup -2/ (1 MeV equivalent fluence). The temperature dependence of the mobility was obtained after irradiation and annealing to 550/spl deg/C for 30 minutes. The maximum value of the mobility, /spl mu//sub max/, with respect to temperature was obtained as a function of fluence. For samples which have been irradiated and then annealed, /spl mu//sub max/ goes through a maximum at a fluence of 10/sup 13/ cm/sup -2/ and is 10% higher than in the unirradiated samples. At higher fluences, the mobility degrades. We attribute the increase in mobility at lower fluences to a radiation induced order effect. The disappearance of the deep level EL12 could be associated with this effect. At higher fluences where the mobility degrades, we observe by photoluminescence spectroscopy, the gallium vacancy, a point defect introduced by the irradiation, and the transfer of the silicon atom from the gallium site to the arsenic site. This suggests that growth of the gallium vacancy or the silicon at the arsenic site can be associated with mobility degradation. >

Radiation induced carrier enhancement and intrinsic defect transformation in n‐GaAs

Gallium arsenide grown by the metallorganic chemical vapor deposition method and n doped with silicon to a concentration of 1015 cm−3 was exposed to reactor neutron irradiation in the 1012 to 3×1014 cm−2 1 MeV equivalent fluence range. Studies of the defects through deep level transient spectroscopy (DLTS), photoluminescence (PL), and transport measurements on this material indicate correlation between the nature and density of defects, and some of the transport parameters. Contrary to the general perception of degradation of electronic properties of semiconductors on nuclear irradiation, we observe enhancement in some of the electrical/optical properties of GaAs on irradiation at lower fluence levels. These properties degrade on irradiation at higher fluences. The PL intensity of irradiated GaAs increases over the 1×1012 to 1×1013 cm−2 fluence range. At 1×1013 cm−2, the increase in the signal from different PL peaks ranges from 25% to 200%. Similarly, the carrier density of irradiated GaAs, as determined...

Proton radiation damage at low temperature in GaAs and GaN light-emitting diodes

It is shown that the radiation hardness for light emission in light-emitting diodes (LEDs) is substantially improved at low temperatures. This is demonstrated through measurements of light emission at room and low temperatures from amphoteric Si-doped gallium arsenide, gallium arsenide quantum well and gallium nitride quantum well LEDs following proton irradiation at room and low temperatures. The enhanced low-temperature radiation hardness for light emission in these LEDs is explained in terms of an improvement in radiative efficiency due to a reduction of nonradiative transition probability at low temperatures. Further, lattice displacement damage in these devices due to irradiation at room temperature is compared with the corresponding damage at low temperatures. Our results show that the amount of lattice damage is dependent on irradiation temperature.

Particle dependence of the gallium vacancy production in irradiated n-type gallium arsenide

The relative introduction rate of the gallium vacancy in n-type GaAs irradiated with /sup 60/Co /spl gamma/ rays, 7 MeV electrons, fusion (14 MeV) and fission (1 MeV) neutrons, protons (0.6 to 200 MeV), deuterons (1 to 10 MeV), /spl alpha/ particles (2.5 to 10 MeV), lithium (5 to 20 MeV) and oxygen ions (10 to 30 MeV) has been determined. Effects of annealing are reported. The measured introduction rates obtained with proton irradiation for energies up to 10 MeV, and for the heavy ions agree reasonably well with Rutherford scattering, NIEL calculations and the TRIM simulation. The results for electron irradiation also agree with the corresponding NIEL calculations.