Contact Resistance of InGaN/GaN Light Emitting Diodes Grown on the Production Model Multi-Wafer MOVPE Reactor
- PDF / 857,244 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 101 Downloads / 174 Views
InGaN/GaN LEDs on multi-wafer MOVPE reactor with high thickness uniformity and the development of a self-aligned LED fabrication process that incorporates a thin current spreading layer. We also present detailed studies on the variation of contact resistance r, of Ni/Cr/Au on p-doped GaN layer versus annealing temperature. EXPERIMENT The InGaN/GaN LED samples were first grown on the Aixtron production model multi-wafer MOVPE reactor. The structures consist of 1.5 ýtm n-GaN layer, a thin InGaN quantum well (4-8 nm), and a 0.5 pim p-GaN cap layer, all grown on 2-inch (0001) sapphire substrates. The growth uses standard precursors including TMGa and TMIn for Group III and NH 3 for Group V elements, and silane and MgCp 2 are used for n-type and p-type dopant sources, respectively. The un-intentionally doped GaN is n-type with carrier concentration - 13x1017/cm 3. The p-type GaN with hole concentration of -5xl017/cm 3 are routinely achieved after post-growth annealing. The thickness uniformity across the diameter of a 2-inch wafer was evaluated using spectral reflectometry measurement in the range of 670 to 1100 nm. Self-Aligned Ni/Au Current Spreading Layer
p - Contact
InGaN/GaN GaN:Mg
A- Ouantum Well
GN:Si ddnft_ý!ý
Fig. 1(a) The schematics of self-aligned InGaN/GaN LED structure. Note that a thin Ni/Au layer is deposited on top of the LED mesa to improve the current spreading.
-n -Contact
Fig. 1(b) SEM of a typical LED structure with a magnified view of the edges of mesa after RIE etching.
The process flow for fabricating a LED with a structure shown in Fig. 1(a), is described as follows. First, a thin current spreading layer consists of Ni/Au (40 Am40 T) was deposited on the entire wafer using electron beam evaporation. The estimated optical absorption due to the current spreading layer is - 38% [4]. The sample is thermally annealed at 5000C under N2 ambient for 100 seconds. The LED mesa is then defined by a photoresist pattern. This is followed by ion milling to remove all except the current spreading layer, and Reactive Ion Etching (RIE) in a mixture of Cl 2 and BC13 to remove the GaN all the way to the n-layer. In this way, the current spreading layer and the mesa are formed in a self-aligned way. Figure 1(b) shows the SEM micrographs of the RIE etched surface around the LED mesa. Both the
bottom and lateral surfaces of the etched mesa appear to be very smooth. Finally, Ni/Cr/Au (150 ,150 A/1200 A) were e-beam evaporated to form both n and p-type contact pads. LEDs without current spreading layer are also fabricated for comparison. By varying the In composition and the thickness of the InGaN quantum well, LED emission from 420 to 490 nm were obtained. To investigate the contact resistance to p-type GaN, a rectangular shaped, mesa isolated transmission line (TL) structure was fabricated. The width of the TL pattern is 300 Pam while the spacing between adjacent Ni/Cr/Au contacts are varied from 5 to 50 p.m at increments of 5 p~m. The TL pattern is defined using RIE etching similar to the LED mesa etching.
Data Loading...
