Analyst Katy Huberty of Morgan Stanley remains bullish on Apple Inc. stock as a survey found implied 15 million surge in iPhone sales in the U.S. by 2015 - a note to investor with AppleInsider revealed.

Morgan Stanley raised its price target on shares of Apple from $630 to $690 with the assumption that the company is gearing towards a gross margin increase of 39 per cent, trading at 15 times price-to-earnings.

According to Huberty's survey data, the 15 million predicted surge in sales will be obtained if the company launch a larger 5-inch display screen, slated to be the rumoured iPhone 6. The company could ship a grand total of 66 million 5-inch iPhones as compared to estimated 51 million units if the iPhone size remains the same.

The survey found that 47 per cent of smartphone customers prefer an iPhone with larger size among other smartphones provider. If launched, the larger iPhone could snatch buyers away from choosing Samsung's Galaxy phones which claim to 'fame' is the larger displays. Huberty noted that carriers for Apple devices will also benefit from this as users are expected to upgrade and switch to wireless providers that will offer the larger iPhone; case at point - Sprint will be negatively affected with this as T-Mobile will make better.

The survey also highlighted that eight out of ten iPhones will be sold to existing iPhone users in 2015 as old-time users remain faithful to the brand. Ninety-per cent of existing users will still buy their smartphones from Apple.

"Current iPhone owners value the software, brand, ability to synchronize to other Apple devices, and the App Store's quality and size more than the average user, which drives Apple's high loyalty rating. The top criteria across all brands are longer battery life, software/operating system, and lower price, though current Apple users think price is of lesser importance," Huberty wrote in the report.

Meanwhile, according to a note from analysts at UBS Research obtained by MacRumors, Apple's supplier of sapphire crystal GT advanced has started shipping the material to the Apple manufacturer in China .Apple is reportedly using sapphire crystal displays for the rumoured iPhone 6.

"We estimate GT's shipment to China was only about $1M worth of sapphire last month. We believe this Apple partner needs to be receiving sapphire totalling about $50M+ per month to confirm that the GT Arizona fab is running at close to full utilization," the UBS note said.