If I cast a Lashweed Lurker using the emerge ability, can I put the creature I'm sacrificing on top of my library?
No, the creature that you sacrificed is in your graveyard and not a valid target for that ability.
601.2h The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can’t be paid.
Example: You cast Altar’s Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell’s total cost is “locked in” before payments are actually made, you pay {B}, not {1}{B}, even though you’re sacrificing the Familiar.
601.2i Once the steps described in 601.2a–h are completed, effects that modify the characteristics of the spell as it’s cast are applied, then the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell’s controller had priority before casting it, he or she gets priority.
The "When you cast" ability doesn't even trigger until step 601.2i. Meanwhile, the creature you sacrificed has been in the graveyard since step 601.2h.
110. Permanents
110.1. A permanent is a card or token on the battlefield. A permanent remains on the battlefield indefinitely. A card or token becomes a permanent as it enters the battlefield and it stops being a permanent as it’s moved to another zone by an effect or rule.
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Why bother with mere rulings when so many answers can be found in the Rules?
No, the creature that you sacrificed is in your graveyard and not a valid target for that ability.
The "When you cast" ability doesn't even trigger until step 601.2i. Meanwhile, the creature you sacrificed has been in the graveyard since step 601.2h.