Electroluminescence and Carrier Transport in Leds Based on Silicon-Rich Silicon Oxide
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Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
EXPERIMENTAL Pre-anodizationProcessing The initial substrates are (100) p-type crystalline Si with a resistivity of - 10 D2 cm. A high dose BF2 implant is done on the backside of the substrate to provide a low resistance contact for anodization. A 1500A Si3N4 film is deposited via low pressure chemical vapor deposition (LPCVD). The frontside of the wafers is then patterned with the first masking level (active), which defines the areas of local anodization. The Si3N4 film is removed via reactive ion etching (RIE) to open windows to the silicon substrate. The film is also completely removed from the backside of the wafer using the same method. A second BF2 implant (dose = 1015 cm- 2 ) is done in the active regions. After photoresist removal and a cleaning procedure, the wafers are annealed at 1000°C for 15 min in N2 to activate the impurities. The wafers are then ready for anodization.
AAl AA
Light output Poly Si (n1+)
Light output
Al
A L
Si 3N 4
Transition layer 0
SRSO
(active laver) •acti•c-Si
(p-type)
c-Si (p+)
Al
Figure 1 Complete device structure (metal/heavily doped /c-Si substrate/high porosity well (active layer)/low
porosity well (transition layer) /insulating layer/ poly Si/ Metal)
Anodization and Annealing A 10 Q cm p-type crystalline silicon wafer with a heavily doped p+ surface layer is anodized in an HF:H20:C2H5OH (1:1:2) solution under a constant current density J = 15 mA/cm 2 . The p+ region is transformed into a mesoporous layer (porosity - 40%) whereas the underlying ptype silicon is transformed into a nanoporous silicon layer (porosity - 75-80%). Annealing at 800-9000C is performed in a dilute oxygen ambient (10% 02 in N2). Hydrogen on the surface of the Si nanoclusters is replaced by oxygen, changing the surface coverage of Si grains from Si-H to Si-O bonds. The material produced by controllable thermal oxidation of PSi can be described as silicon-rich silicon oxide (SRSO). Cathode and ContactFormation: Poly-Si is chosen as the top contact material (Fig. 3). A 0.3 Rm poly-Si film, which is transparent in the red-IR part of the spectrum, is produced via LPCVD performed at 610oC. This process does not damage samples which were previously annealed at 800-9000C. The
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poly-Si layer is patterned photolithographically to define the device contact region. To provide efficient electron injection during device operation, the poly-Si layer is heavily doped (n+) by phosphorus ion implantation with a dose of 1015 cm-2 at an energy of 50 KeV, followed by an activation anneal which produces a low resistivity polysilicon contact (p < 0.1 Q cm). Aluminum contacts are then made to the n+ poly-Si film and the bulk substrate. The gap between top Al contact and the border of the active region is - 10 Atm. The complete device structure can be seen in fig. 1. 8
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