Cubeを接地した状態で回転させる方法について

RotateCubeTest.cs

using System.Collections;
using UnityEngine;

public class RotateCubeTest : MonoBehaviour
{
    const float RotatingSpeed = 0.2f;
    const float RotatingAngle = 90f;
    Vector3 halfSize;
    float time = 0f;
    Vector3 axis = Vector3.zero;
    Vector3 point = Vector3.zero;

    void Awake()
    {
        this.halfSize = this.transform.localScale / 2f;
    }

    void Update()
    {
        if (this.point != Vector3.zero) { return; }
        if (Input.GetKey(KeyCode.UpArrow))
        {
            this.axis = Vector3.right;
            this.point = this.transform.position + new Vector3(0f, -this.halfSize.y, this.halfSize.z);
        }
        else if (Input.GetKey(KeyCode.DownArrow))
        {
            this.axis = Vector3.left;
            this.point = this.transform.position + new Vector3(0f, -this.halfSize.y, -this.halfSize.z);
        }
        else if (Input.GetKey(KeyCode.LeftArrow))
        {
            this.axis = Vector3.forward;
            this.point = this.transform.position + new Vector3(-this.halfSize.x, -this.halfSize.y, 0f);
        }
        else if (Input.GetKey(KeyCode.RightArrow))
        {
            this.axis = Vector3.back;
            this.point = this.transform.position + new Vector3(this.halfSize.x, -this.halfSize.y, 0f);
        }
        if (this.point != Vector3.zero)
        {
            StartCoroutine(this.StartRotate());
        }
    }

    IEnumerator StartRotate()
    {
        float nowAngle = 0f;
        float addAngle = (RotatingAngle / (RotatingSpeed * 60f));
        while (this.time <= RotatingSpeed)
        {
            nowAngle += addAngle;
            if (nowAngle <= RotatingAngle)
            {
                this.transform.RotateAround(this.point, this.axis, addAngle);
            }
            this.time += Time.deltaTime;
            yield return null;
        }
        this.time = 0f;
        this.axis = this.point = Vector3.zero;
        yield break;
    }
}