Here we present two techniques, which have advantages in the perovskite single crystal devices. First, we demonstrate modulation-doped layer growth and double heterostructure using a millimeter-sized hybrid halide perovskite crystal as a substrate. We show that previously known limiting factor of halide ion inter-diffusion can be constrained to few microns by (1) using low halide composition gradient and (2) adjusting solution concentrations just above the critical super saturation. In the solvo-thermal growth process, our layer growth time could be conveniently extended as necessary to grow a uniform layer, with only ~5 µm inter-diffusion region. This is a significant improvement compared to few seconds dipping time previously reported for a rapid ion exchange process without any layer growth. The growth of CH3NH3PbBr3 layer on top of CH3NH3Pb(Br0.85Cl0.15)3 bulk substrate is studied for different growth times to obtain up to 30µm layer thickness. Ion diffusion profile, layer thickness and crystallographic orientation have been characterized by cross sectional characterization using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Electron Back-Scattering Diffraction (EBSD) . With this advancement, we are able to grow two consecutive perovskite layers to create a double heterostructure for the first time. Second, we demonstrate an as-grown milliliter-sized perovskite bulk crystal light emitting device. This device can be easily lighten up at low voltage (6-20 V) and at slightly low temperatures than room temperature (160-230 K). We are aiming to integrate both technologies with further optimization to produce efficient, pure-color perovskite light emitting devices for entire visible spectrum with low-cost and simple infrastructure.