Full sequence of letters

Given a string of two letters, where first one occurs before the second in the alphabet, write a method that returns full sequence of letters starting from first and ending at the second one.
Expected input and output
FullSequenceOfLetters("ds") → "defghijklmnopqrs" FullSequenceOfLetters("or") → "opqr"
using System;

namespace CSharpExercises.Exercises.Loops
{
    class FullSequenceOfLettersTask
    {
        static string FullSequenceOfLetters(string word)
        {
            string fullSequence = string.Empty;
            for (int i = word[0], j = 0; i <= word[1]; i++, j++)
            {
                fullSequence += (char)(word[0] + j);
            }

            return fullSequence;
        }

        static void Main(string[] args)
        {
            Console.WriteLine(FullSequenceOfLetters("bg")); // bcdefg
            Console.WriteLine(FullSequenceOfLetters("xy")); // xy            
            Console.WriteLine(FullSequenceOfLetters("az")); // abcdefghijklmnopqrstuvwxyz
        }
    }
}