Dependence of aging on inhomogeneities in InGaN/AlGaN/GaN light-emitting diodes
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ameters of 3 LEDs chousen for careful measurements are given in Table 1. Spectral maxima (hωmax), power efficiencies η%, the hole concentration p on the p- side of the junction, a width of the space charge region w are given in the Table 1 with parameters describing the exponential tails of the main spectral band. The long wavelength side parameter is E0; the short wavelength one is E1 = m⋅kT. The distributions of charges in the space charge regions NA- (w) and p-values for Q, N and P LEDs derived from dynamic capacitence measurements see Fig. 3 in [7]. The LEDs Q and P had more compensated space charge regions; the LED N had the lowest value of the width of the space charge region and clearly pronounced tunnel radiation band with hωmax ≈ eU. Parameter P, x1017cm-3 w, nm η%(10 mA) η%(0.1 mA) hωmax (10 mA), eV W, mW (10 mA) E0, meV E1=mkT, meV
Table I. Parameters of LEDs before aging. Q N 6 3 200 120 2.27 2.41 1.65 0.006 2.485 2.486 0.84 0.7 56.7 59.1 33.2 31.8
P 2 130 2.57 0.001 2.500 0.63 56.1 33.6
The first experiment of LED’s aging was performed in the same conditions as in [4-6] that is at J = 80 mA. The LEDs at this current strongly degraded during 24 hours. That is why the conditions were choused by gradual changing of working currents in the range J = 30 ÷ 60 mA during 168 hours (see Table 2). Spectra and quantum efficiency of the LEDs were measured in the range J = 10 µA ÷ 60 mA each 24 hours. Table II. Conditions of aging. Time, hours 0-24 24-48 48-72 Reverse current, 1 mA shift with current in the range hωmax ≈ 2.36÷2.50 eV. The tunnel band at low currents had maxima according to the applied voltage: hωmax ≈ eU =1.92÷2.13 eV. It is essential that we could trace changes of
F99W11.25
spectral form during aging. The exponents of spectral tails (E0 and E1 = m⋅kT, see a discussion lower) change during the aging. Spectral intensities at low currents fell down during aging. This is more pronounced for the LED P (the lowest current at which the spectra could be measured was 60 µA before aging and 1 mA after aging). 2,48 eV, 4,55 r.u. 10
0,01
2,37eV, 6,80E-5 r.u.
Fit
1E-3
10 mA 1 mA qU=2.02 eV 48 mkA 23 mkA
1E-4 1E-5 1E-6
10 mA 1 mA 0.5 mA 0.1 mA 60 mkA
1 0,1
Intensity, relative units
1 0,1
Intensity, relative units
2.50 eV, 5.29 r.u. 10
10 mA 1mA 0.1 mA 48 mkA 23 mkA
1E-7
0,01 1E-3 1E-4
Fit 10 mA 1 mA qU=2.08 eV 0.5 mA 60mkA
1E-5 1E-6 1E-7
2.36 eV, 9.64e-6 r.u.
2.36 eV, 9.42e-7 r.u.
1E-8
qU=1.94 eV
1E-8
2,39 eV, 3,90E-7 r.u.
qU=2.00 eV
1E-9
1E-9 1,6
1,8
2,0
2,2
2,4
2,6
1,6
2,8
1,8
2,0
2,2
Figure 1a. Spectra of LED N and their approximations before degradation.
1E-3 1E-4 1E-5 1E-6
10 1
2,37 eV 5,6E-5 r.u.
Fit 10 mA qU=2.1 eV 1 mA 51 mkA
1E-7 2,37 eV 4,25E-7 r.u.
1E-8
0,01 1E-3 1E-4
Fit 10 mA 1.165 mA
2,36 eV 1,14E-5 r.u.
qU=2.12 eV
1E-5 1E-6 1E-7
2,35 eV 1,14E-6 r.u.
1E-8
qU=1.96 eV
qU=2.045 eV
1E-9 1,6
10 mA 5 mA 2 mA 1.3 mA 1.165 mA
0,1
Intensity, relative units
Intensity, relative units
0,1 0,01
2,8
2,49 eV 3,68 r.u.
2,48 eV 4,40 r.u.
10 mA 1 mA
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