# III-Nitride Multi-Quantum-Well Light Emitting Structures with Selective Carrier Injection
## 1 Introduction  
Inhomogeneous carrier injection and an uneven population of optically active quantum wells unfavorably affects the performance of multiple-quantum-well (MQW) light emitters [1]. In III-nitride based LED structures, inferior hole transport is the commonly accepted source of inhomogeneous injection [2]. The excessive depth of InGaN quantum wells employed in visible-range light emitters and related increases in quantum well (QW) population capacity, however, are equally important causes of injection non-uniformity [3].  
In light emitters based on other III-V materials (for instance, mid-infrared lasers), shallow and relatively wide quantum wells can be uniformly populated by efficient carrier exchange with highly mobile free-carrier subsystems [4]. Net confined charges of such quantum wells in LED operation mode are usually small. On the contrary, in deep-QW III-nitride heterostructures, the exchange between mobile and confined carriers is strongly biased toward the capture process. The latter is adversely affected by ballistic overshoot phenomena and becomes increasingly inefficient for narrow InGaN quantum well layers. As a result, the final established balance between the slow carrier capture and fast intra-QW recombination keeps the dynamic quantum well populations in profoundly away from equilibrium, and in case of different rates of electron and hole capture, also increases the net confined QW charge. In excessively deep green or red-emitting QWs, the deviation of confined carriers from an equilibrium with a mobile carrier subsystem becomes progressively stronger [5].  
Since electrons and holes are injected from the opposite sides of the diode structure, their uneven distribution across the active region further upholds the inequality between quantum-confined electron-and-hole populations, which is especially noticeable in marginally located n-side and p-side quantum wells [6]. Residual confined charges, in turn, distort the active region's potential profile, and depending on the structure polarity, can either promote the quantum well ballistic overshoot [7]or enhance the carrier capture [8]. In c-plane polar III-nitride MQW LEDs, the resulting injection distribution usually favors the marginal p-side QW [9] which is amply populated with low-mobility holes, and therefore, always sustains enough net positive charge to attract mobile electrons for capture and recombination.  
The mutual interdependence between QW populations and MQW injection distribution underlines the complexity of MQW device design and calls for application-dependent active region optimization. A strong interaction between different design elements makes MQW light emitter a nonlinear distributed system [1] so that all the active region constituents should be optimized simultaneously. In our approach, MQW injection rebalancing for a particular application is accomplished by including into the active region specially designed intermediate carrier blocking layers (IBLs) [10]; see Figure 1. The IBL design is comprised of both compositional and doping optimization and should be correlated with active quantum well compositions. The optimal IBL layout would also essentially depend on the required LED operational conditions like nominal bias voltage or injection current. This approach has been successfully applied to achieve the first full-color tunable monolithic LED covering complete RGB gamut [11; 12], and recently, has been used to implement a white-color LED with tunable color temperature [13]. In this paper, we demonstrate yet another example of the IBL design of III-nitride MQW LED active region featuring a uniform population of active quantum wells at LED operational voltage.