InGaAs nanostructures grown by droplet molecular beam epitaxy

InGaAs ring-shaped nanostructures are fabricated by droplet epitaxy technique using solid source molecular beam epitaxy (MBE). InGa droplet forming conditions have been varied in order to investigate the effect of substrate temperature during InGa deposition and deposited amount of indium (In) and Gallium (Ga) on InGaAs ring-structures. It was found that increasing substrate temperature during InGa deposition resulted in InGaAs rings larger size but lower density since greater diffusion length of metallic In and Ga atoms responsed to 2-dimensional expansion of InGa droplets and coalescence of neighbouring droplets. Also, the increasing InGa amount deposited would result in InGaAs rings larger size. The ring density is also increased when increasing InGa amount between 2-3 ML. However, when increasing InGa amount to 4 ML, the density becomes to decreased. The density is increased again while increasing InGa amount to 5 ML. It was supposed that decrease of ring density was caused from formation of InGa full-layer instead of individual droplets when greater amount of InGa was deposited. Subsequently, newly supplied InGa adatoms would from the droplets above the layer resulting in renewal of the number of the droplet formed. For photoluminescence (PL) measurement, the nanostructures were repeatedly grown under selected droplet forming condition with additional 100-nm GaAs capping layers including GaAs layers grown by migration-enhanced epitaxy and conventional GaAs layers. The analytical optical properties of InGaAs rings are confirmed by the PL spectra of capped InGaAs ring-structures at 77 K. The PL result indicates the high quality crystal. However, PL intensity was found to be low resulting from low density of the ring structures (10 [superscript 8] cm [superscript -2]).

本研究采用固态源分子束外延（MBE）技术，通过液滴外延法制备了InGaAs环形纳米结构。为探究铟镓沉积阶段的衬底温度、铟（In）与镓（Ga）沉积量对InGaAs环形结构的影响，我们调控了铟镓液滴的形成条件。研究发现，提高铟镓沉积过程中的衬底温度，会使InGaAs环形结构的尺寸增大但密度降低，这是因为金属铟与镓原子的扩散长度增加，促使铟镓液滴发生二维扩展以及相邻液滴的聚并。同时，增加铟镓沉积量可使环形结构的尺寸增大；当沉积量处于2~3单层（ML）区间时，环形结构的密度随沉积量提升而升高，但当沉积量增至4单层时，密度反而下降；而当沉积量进一步提升至5单层时，密度再次升高。我们推测，当沉积量过大时，铟镓会形成完整液层而非独立液滴，这是导致环形结构密度下降的原因。随后，新供给的铟镓吸附原子会在该液层表面形成新的液滴，使得液滴的生成数量发生重置。为开展光致发光（PL）测试，我们在选定的液滴形成条件下重复生长了该纳米结构，并额外制备了100 nm厚的GaAs覆盖层，其中包含采用迁移增强外延（MEE）生长的GaAs层以及常规GaAs层。通过77 K下覆盖型InGaAs环形结构的光致发光光谱，验证了InGaAs环形结构的光学特性。PL测试结果表明该结构具备高质量的晶体品质，但由于环形结构的密度较低（10⁸ cm⁻²），其PL强度相对较弱。