Rectangular Cube is sinking in floor rotating at 90 degree

Okay so I am trying to make a game like "bloxorz" in which a rectangular cube is rotated at 90 degrees. But when I try to implement it the cube is half sinking to the floor at 90 and then fully sink at another 90 degree. And I am using box collider and script that is given below. And I also attached an empty gameobject to parent cube. I am thinking that rigid body will help but I am new to unity so don't know how to implement it. And also that cube is working perfectly but only rectangular cube has this problem. Please help I am stuck at this.

Code for Rectangular Cube:

 private var ismoving : boolean = false;  
 private var startY : float = 0;  
 var cubeSpeed : float;  
 var cubeSize : float;
   
 function Update ()   
 {  
   
     if (Input.GetKeyDown("up") && ismoving == false)  
     {  
         ismoving = true;  
         transform.Find("targetpoint").Translate(0, -cubeSize/2 , cubeSize/2);  
         StartCoroutine(DoRoll(transform.Find("targetpoint").position, Vector3.right, 90.0f,cubeSpeed));  
     }   
     if (Input.GetKeyDown("down") && ismoving == false)  
     {  
         ismoving = true;  
         transform.Find("targetpoint").Translate(0, -cubeSize/2, -cubeSize/2);  
         StartCoroutine(DoRoll(transform.Find("targetpoint").position, -Vector3.right, 90.0f,cubeSpeed));  
     }  
     if (Input.GetKeyDown("left") && ismoving == false)  
     {  
         ismoving = true;  
         transform.Find("targetpoint").Translate(-cubeSize/2, -cubeSize/2, 0);  
         StartCoroutine(DoRoll(transform.Find("targetpoint").position, Vector3.forward, 90.0,cubeSpeed));  
     }  
     if (Input.GetKeyDown("right") && ismoving == false)  
     {  
         ismoving = true;  
         transform.Find("targetpoint").Translate(cubeSize/2, -cubeSize/2, 0);  
         StartCoroutine(DoRoll(transform.Find("targetpoint").position, -Vector3.forward, 90.0f,cubeSpeed));     
     }  
 }  
   
 function DoRoll (aPoint, aAxis, aAngle, aDuration) {    
   
  var tSteps = Mathf.Ceil(aDuration * 30.0);  
  var tAngle = aAngle / tSteps;  
  var pos : Vector3; // declare variable to fix the y position  
    
 // Rotate the cube by the point, axis and angle  
  for (var i = 1; i <= tSteps; i++)   
   {   
     transform.RotateAround (aPoint, aAxis, tAngle);  
     yield WaitForSeconds(0.0033333); 
     Debug.Log("Tsteps : " + tSteps); 
   } 
    
 // move the targetpoint to the center of the cube   
    transform.Find("targetpoint").position = transform.position;  
   
 // Make sure the y position is correct 
    pos = transform.position; 
    pos.y = startY;
    transform.position = pos;  
    
       
 // Make sure the angles are snaping to 90 degrees.       
    var vec = transform.eulerAngles;  
    vec.x = Mathf.Round(vec.x / 90) * 90;  
    vec.y = Mathf.Round(vec.y / 90) * 90;  
    vec.z = Mathf.Round(vec.z / 90) * 90;  
    transform.eulerAngles = vec;  
  
       
 // The cube is stoped  
    ismoving = false;       
 }  

Code for child GameObject:

  var Radius : float;
     
     function Update () {
         transform.rotation = Quaternion.identity;
     }
     
     function OnDrawGizmos(){
         Gizmos.DrawSphere (transform.position, Radius);
     }