Crystalline Structure, Optical Properties and Band Gap Nature of Semiconductor Ca2Si Films on Al2O3 Substrate

Abstract

Аннотация. Методом преобразования расходуемого 2D-слоя Mg 2 Si в затравочный слой Ca 2 Si были впервые сформированы нанокристаллические пленки Ca 2 Si на Al 2 O 3 (0001) с предварительным формированием аморфного 2D-слоя Si. Установлено, что затравочный слой Ca 2 Si на сапфировой подложке позволяет выращивать ориентированные пленки Ca 2 Si методом молекулярно-лучевой эпитаксии при температуре 250 о С, для которых наблюдается одно эпитаксиальное соотношение: Ca 2 Si(211)/Al 2 O 3 (0001). Исследования оптических свойств и параметров зонной структуры Ca 2 Si на сапфире выявили природу прямого фундаментального перехода с энергией 0.88±0.01 эВ. Установлено, что в зонной структуре Ca 2 Si наблюдаются четыре прямых межзонных перехода: 0.88, 1.16, 1.49 и 1.61 эВ с увеличивающейся силой осциллятора. При энергиях фотонов от 0.78 эВ до 0.88 эВ основной вклад в поглощение вносит хвост Урбаха на дефектах в нанокристаллах Ca 2 Si. Поглощение на межзеренных границах наблюдается при энергиях фотонов от 0.6 эВ до 0.78 эВ, а на свободных носителях -при энергиях менее 0.6 эВ. Полученные результаты важны для оптоэлектроники в ИК области спектра. Ключевые слова: 2D Mg 2 Si, сапфир, шаблон, пленки Ca 2 Si, кристальная структура, оптические функции, фундаментальный прямой переход.

Ca silicide films—promising materials for silicon optoelectronics

“…26) For the first time, a nanocrystalline (NC) Ca 2 Si film was grown on a transparent Al 2 O 3 (0001) substrate with sequential formation of an amorphous 2D Si layer, a sacrificial 2D Mg 2 Si layer and a Ca 2 Si seed layer. 23,24,27) One Ca 2 Si(211)/Al 2 O 3 (0001) epitaxial relationship was found in this NC film. Moreover, individual CaSi nanocrystals were detected as well with the CaSi(001)/Al 2 O 3 (0001) relationship.…”

“…CaSi and CaSi 2 films were grown on silicon and sapphire substrates using the sacrificial-template method, which is described in detail elsewhere, with the selection of magnesium, calcium, and silicon deposition rates. 21,27) CaSi and CaSi 2 films were grown on both types of substrates using reactive deposition epitaxy (RDE) and molecular-beam epitaxy (MBE) methods with the selection of the substrate temperature, Ca deposition rate, and the ratio of Ca to Si fluxes to implement singlephase growth during MBE growth. 34,36,40) Growth of calcium silicides on silicon and sapphire with structures Ca 5 Si 3 and Ca 14 Si 19 were carried out according to the original method through the formation of calcium monosilicide (by MBE or RDE methods) followed by an increase in the amount of Ca by RDE with the selection of the substrate temperature from 500 °C to 700 °C to convert CaSi to Ca 5 Si 3 or Ca 14 Si 19 .…”

“…Studies of the optical properties and parameters of the band structure of Ca 2 Si on sapphire revealed a direct fundamental transition of 0.88 ± 0.01 eV in addition to three more direct interband transitions of 1.16, 1.49, and 1.61 eV with appreciable oscillator strength. 27) At photon energies from 0.78 to 0.88 eV, the main contribution to absorption is due to the Urbach tail on defects in Ca 2 Si nanocrystals, while absorption at grain boundaries occurs at photon energies from 0.6 to 0.78 eV. 26) It is also assumed that the contribution of the CaSi phase is associated with absorption by free carriers injected into the Ca 2 Si film at photon energies below 0.6 eV.…”

“…Since theoretical [13][14][15][16]18) and experimental 21,25,27) investigations of semiconductor Ca 2 Si have revealed its complex electronic structure, the temperature dependences of the spectral photoconductivity were studied for the first time in the temperature range from 10 to 300 K to confirm the photogeneration of carriers and determine the nature of the interband transition in the Ca 2 Si epitaxial film grown on a Si (111) substrate. 28) Since the red boundary in the photoconductivity spectrum is proportional to the absorption coefficient and depends on the energy position of the fundamental interband transition, 26) using three thermodynamic models, the existence of a direct fundamental transition with E g = 1.195 eV at 0 K was established, and the experimental dependence E g (T) was plotted.…”

“…[29][30][31] This interband transition at 300 K (1.163 eV) is close to the value (1.195 eV) determined at 300 K from optical spectroscopy data. 21,25,27) In addition, the effective phonon energy (〈E ph 〉 = 48.3 ± 1.8 meV), the dimensionless electron-phonon coupling constant (S = 1.819 ± 0.17), the Einstein (Ξ = 561 K) temperature, and the hole mobility μ = 98 cm 2 V −1 × s −1 at room temperature were determined.…”