Intersubband Transition in AlGaN/GaN step quantum wells at 3–5 μm

Abstract

We demonstrate intersubband absorptions and photocurrent response at wavelength of 3-5μm in nitride-based semiconductor step quantum wells. The structures consist of a 1.8nm thick Al 0.5 Ga 0.5 N barrier, a 1.8nm thick GaN well and a 16nm thick Al 0.25 Ga 0.75 N step barrier. With this approach, one can create a virtually flat band potential energy profile in the step barrier layers, which is confirmed by analysing the temperature dependence dark current results.GaN based intersubband (ISB) devices has attracted much research attention due to the benefits of large conduction band offset, ultra-fast electron relaxation time, and UV-region capability. Among them, traditional c-plane AlGaN/GaN multiple quantum wells (MQWs) structures are widely used. However, the polarization induced internal electric field reduces not only the ISB transition probability but also the photocurrent transportation, which significantly reduces the devices' performance.Some groups eliminated the influence of internal electric field by non-polar GaN based quantum structures [1] and cubic ones [2] . To overcome the poor growth quality of samples in these methods, we use step quantum wells (SQWs) structure in which the internal electric field is proved almost completely eliminated. By this structure, we realized the detection with photoabsorption and photocurrent at wavelength of 3-5μm.We designed and grew Al 0.5 Ga 0.5 N/GaN/Al 0.25 Ga 0.75 N SQWs on c-plane sapphire substrate. The structure was designed based on solving self-consistent Schrödinger-Poisson equations. It shows that when we use a structure of Al 0.5 Ga 0.5 N/GaN/Al 0.25 Ga 0.75 N (1.8nm/1.8nm/16nm, barrier/well/step barrier) SQWs, the internal electric field of step barrier layer can be completely erased. The conduction band structure and energy levels of the designed structure are shown in Fig.1. There are several separated energy levels: the ground state (e 1 ) and the quasi-continuum states (e 2 -e 11 ). High resolution TEM (Fig.2) of the grown sample shows that each layer thickness is 7ML/6ML/65ML, respectively. By simulation of XRD diffraction peaks, we estimated the Al component of each layer, which is in accord with the designed structures.Afterwards the temperature dependence dark current is measured. It shows the symmetric IV characteristic dark current at high temperature (>100K) and the asymmetric one at low temperature. At high temperature, the ground state carriers jump up to quasi-continuum states by thermionic emission. The elimination of internal electric field at wide barrier layer results in the symmetric IV characteristic dark current if we ignore the influence of narrow barrier for it is narrow enough for tunneling. At low temperature, the ground state carriers are confined at the quantum wells.They are very hard to tunnel to the adjacent well at low bias because of the wide step barrier blocking, while they can tunnel at high bias. The asymmetric structure results in the asymmetric tunneling method, and then the asymmetric IV characteristic dark current....