What quality metrics scrum team can use to improve the product quality?
E.g. Defect density. How it is calculated in scrum projects?
Are there any other Quality Metrics which can be followed?
What quality metrics scrum team can use to improve the product quality?
No metrics will improve product quality, only observing a satisfactory Definition of Done will achieve that. The best measurements are usually taken as closely as possible to the time and place of work being carried out.
A metric, by itself, will not improve quality. A team may be able to use metrics to understand the quality of the product as well as the processes used to build the product. I'm having trouble thinking of a quality metric that wouldn't be useful to a team just because they were using Scrum. The hardest to use would be phase-based process quality metrics, since there are often no clear-cut definitions of phases in Scrum.
Whenever thinking about metrics, I find it best to start with a problem. Based on the problem, there may be data that is necessary to determine if there is a problem and how significant it is. The best metrics to demonstrate the existence of a problem arise from the problem itself.