Contact Issues of GaN Technology
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Downloaded from https://www.cambridge.org/core. IP address: 93.179.89.78, on 17 Apr 2019 at 04:37:45, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002234
to the presence of the piezoelectric donor charge at the AlGaN/GaN interface[2]. Internal photo emission is an alternative way to determine the Schottky barrier height of a metal/AlGaN/GaN heterostructure. In our laboratory, we have measured the barrier height of Ni of Al0.15Ga0.85N (300 Å and 500 Å)/GaN using the internal photo emission technique and obtained a barrier height of ~1.30 eV, independent of the AlGaN layer thickness[3]. This value, well within the experimental scattering range, is considered to be consistent with the value obtained on bulk Al0.15Ga0.85N samples. As the mole fraction of Al in the AlGaN top layer changes to 30%, the barrier height is seen to increase further to ~1.56 eV (see Table 1). These results suggest that the Schottky barrier of HFET structures is largely determined by the upper most AlGaN layer and that the barrier height appears to increase between 0.25 to 0.3 eV for every 15% increment in Al mole fraction in the AlGaN layer up to 30 % of Al. More work is needed to correlate the barrier height and the Al mole fraction in detail. Table 1. Summary of Schottky barrier height(a) Ideality Metal Ni Ni Ni Ni Ti Ti
Material
n factor (b)
Bulk AlGaN GaN
qφb (I-V) (e)
(eV)
qφb (I-V) (f)
(eV)
qφb (C-V) (eV)
qφb (photo) (eV)
1.23
1.03
1.25
1.26
1.28
1.14
0.84
0.95
0.96
0.91
(c)
HFET
1.31
(d)
HFET
1.56 (b)
Bulk AlGaN GaN
1.08
0.79
0.84
1.10
1.08
0.60
0.65
0.68
(a)
The data (I-V and C-V) are the average value from 15 diodes; the standard deviation is about 0.05 for both of the n factor and the barrier heights. (b)
Al mole fraction was 15% in the AlGaN samples.
(c)
Al0.15Ga0.85N (300 Å or 500 Å)/3µm undoped GaN.
(d)
Al0.3Ga0.7N (500 Å)/3µm undoped GaN.
(e)
Calculated from Equation 1;
I = AA*T 2 e − qφ b / kT (e qV / nkT − 1) (f)
(1)
Calculated from Equation 1 and corrected by equation 2;
φbc = nφb − (n − 1)
kT Nc ln q Nd
(2)
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We have also been investigating the ohmic behavior on the III-V nitrides. Of particular interest to us was understanding how to fabricate low resistance and reproducible ohmic contacts on HFET structures. During the course of the study, we found that the ohmic behavior varied significantly from one wafer to another, even though these wafers were nominally the same, i.e., n- Al0.15Ga0.85N (300 Å)/ Al0.15Ga0.85N (30 Å, undoped)/i-GaN(1 µm). Table 2 shows the results of the measured contact resistivity on four different wafers with nominally the same structure. The nsµ product, extracted from Hall effect measurements, is the usual parameter that character
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