Role of Nanopipes in Degradation of AlGaN/InGaN/GaN Devices Operating at High Voltage
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1. Are Nichia LEDs better from crystallographic point of view than typical nitride material grown on sapphire in other laboratories? 2. How does the presence of defects affect the LED lifetime under close-to-normal operating conditions? 3. What predictions can be made with respect to possible problems with reliability of
nitride-based diode lasers? As described in more detail below, we found out that the LEDs were made of material containing enormous density of extended defects that in other III-V compounds would only qualify as reject. Yet, the LEDs seemed to perform very well, and our extensive life
testing, including devices driven cw at currents twice exceeding the maximum rating and at temperatures surpassing the maximum allowed in the manufacturer specifications, failed to produce any damage inside the semiconductor material. A more detailed recent account of that part of our work can be found in [4]. The answer to the third question is the subject of this paper. We show that our early predictions, formulated back in 1995 [5], were confirmed by recent observations on both the operating laser devices and the nitride material itself. DEVICES UNDER STUDY
Our early studies began with investigations of double-heterostructure (DH) Nichia NLPB-500 blue LEDs, shown schematically in Fig. 1. More recently, we focused our attention on newer generations of Nichia LEDs of the NSPB and NSPG series with singlep-contact quantum-well (SQW) active region, emitting in the blue and green spectral range. Their structure is very similar to that shown in Fig. 1, except for the obvious change in the Active active-region thickness and compoMg:GaN region sition. Ma:AIGaN ,. Fig. 1. Schematic structure of Nichia NLPB-500 DH blue LED. The layer thicknesses are: GaN buffer - 30 nm; n-type Si-doped layers: GaN - 4 gm, A10 15 sGao.g5N - 150 nm; Ino.06Ga0 .9 4N active region co-doped with Zn and Si - 50 rm; p-type Mg-doped layers: Alo 1:5Gao.g5N - 150 rim, GaN - 500 nm. The ohmic contact materials are: Ti-Al on n-side, and Au-Ni on p-
Zn, Si:lnGaN Si:AIGaN
n-contact
Si:GaN GaN buffer layer Sapphire substrate
side. After Ref 6. HIGH-ELECTRICAL STRESS EXPERIMENTS In order to simulate laser-like conditions, we subjected the LEDs to low-duty-cycle high-amplitude rectangular voltage pulses of 100 ns duration and 1 kHz repetition rate. Even though the LEDs were not packaged for that type of testing and their maximum
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rating under pulsed conditions was only 100 mA, we were able to drive pulses up to -1.5 A in the case of DH LEDs, and as much as 6 A for SQW LEDs, before nearly instantaneous degradation would occur. The corresponding current densities varied between 2 and 7 kA/cm2, thus exceeding the level required to reach threshold in nitride-based lasers. RESULTS Double-heterostructure LEDs
Electrical and optical characteristics of blue DH LEDs were relatively stable up to moderately high-bias pulses. Noticeable changes were recorded at currents approaching 1.5 A. Minor and partially reversible diode damage manifested itself in cha
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