Accessing data from the model class in yii2

In yii2 I have a controller and model built with gii tool. However when I call the model function from the controller, it gives an error.

PHP Fatal Error 'yii\base\ErrorException' with message 'Class 'backend\controllers\TargetCities' not found' 

in C:\xampp\htdocs\ncddp\backend\controllers\TargetCitiesController.php:15

Stack trace:
#0 [internal function]: yii\base\ErrorHandler->handleFatalError()
#1 {main}

This is my Controller.........................................

<?php

namespace backend\controllers;

class TargetCitiesController extends \yii\web\Controller
{
    public function actionIndex()
    {
        return $this->render('index');
    }

    function actionGetcities()
    {
    $prov_code = $_POST['prov_code'];
     $cities = TargetCities::model()->get_cities($prov_code);
     echo $cities;
    }

}

and this is my model...................

<?php

namespace app\models;

use Yii;

/**
 * This is the model class for table "tbl_target_cities".
 *
 * @property integer $id
 * @property string $city_code
 * @property string $kc_classification
 * @property integer $cluster
 * @property integer $grouping
 * @property string $priority
 * @property integer $launch_year
 */
class TargetCities extends \yii\db\ActiveRecord
{
    /**
     * @inheritdoc
     */
    public static function tableName()
    {
        return 'tbl_target_cities';
    }

    /**
     * @inheritdoc
     */
    public function rules()
    {
        return [
            [['city_code', 'kc_classification', 'cluster', 'grouping', 'priority', 'launch_year'], 'required'],
            [['cluster', 'grouping', 'launch_year'], 'integer'],
            [['city_code'], 'string', 'max' => 20],
            [['kc_classification', 'priority'], 'string', 'max' => 100],
            [['city_code'], 'unique']
        ];
    }

    /**
     * @inheritdoc
     */
    public function attributeLabels()
    {
        return [
            'id' => 'ID',
            'city_code' => 'City Code',
            'kc_classification' => 'Kc Classification',
            'cluster' => 'Cluster',
            'grouping' => 'Grouping',
            'priority' => 'Priority',
            'launch_year' => 'Launch Year',
        ];
    }


    public function get_cities(){
        $select = "SELECT * FROM lib_cities where prov_code=$prov_code";
        $query = Yii::app()->db->createCommand($select)->queryAll();
        return $query;
    }
}