Defect engineering of quantum wells in a single semiconductor material
In short: One of the most promising materials for optoelectronics is zinc oxide, with efficient blue and utraviolet light emission. A zinc oxide crystal consists of stacked layers of atoms, and two different stacking sequences known as wurtzite and zincblende can occur. Our study shows that faults in the stacking sequence in fact are minimal segments of zincblende inserted into a wurtzite crystal. A zincblende insertion traps and confines electrons, but not holes, acting as an electron quantum well. Once the insertions of zincblende are controllable, they can be used for tuning the electronic and optical properties of the material.
Most of the widely used semiconductor materials possess periodic crystal structure. However, the arrangement of atoms or molecules in the materials is not perfect: the regular patterns are interrupted by crystallographic defects. Availability of defects modifies significantly the functional properties of the semiconductor or even degrades them completely. Therefore, defects control and fabrication of high quality semiconductor materials is still an essential challenge for material science. However, once the defects incorporation can be controlled, they can be considered as an additional unique advantage for modification of the properties of semiconductors.
One of the most promising materials for optoelectronics is zinc oxide, with emission of light in the ultraviolet and blue region of the spectrum. Furthermore, its uniqe properties ensures very high efficiency of light emission at and above room temperature.
A zinc oxide crystal consists of stacked layers of atoms. Two different stacking sequences are relevant for zinc oxide, the first is periodically repeated every second layer, and the second sequence is repeated every third layer. At normal conditions is the stacking sequence of zinc oxdide of the first type, the so-called wurtzite structure. However, at specific conditions can zinc oxide also exist the other phase known as the zinc blende structure. Simultaneous existence of both phases in the material was almost never observed, in contrast to other materials, like gallium arsenide, for instance. On the other hand, faults in the stacking sequence are often observed in zinc oxide, with irregularity in the planar stacking sequence of atoms in a crystal. Such stacking faults affect the optical properties of the material, and the optical emission spectrum undergoes significant changes with new spectral features appearing. These features has been observed in previous studies, but their origin, however, is often misinterpreted in the literature and and their nature and spectral characteristics has not been studied.
Our study shows that the stacking fault in fact is a minimal segment of zinc blende inserted into a wurtzite crystal (Fig. 1 top). Furthermore, this stacking fault will attract and trap electrons since the potential energy of the electrons is lower the zinc blende structure compared to the wurtize. This potential well is very thin, only a few nanometers wide, implying that the trapped and confined electrons must be described and understood by quantum mechnics. Such so-called quantum wells have a large scientific and technological interest, but they are normally fabricated by combining two different semiconductor materials.
In order to study the above described phenomena, we have prepared nanostructures of zinc oxide nanostrucures with high density of stacking faults (up to 106 per centimeter). The stacking faults were of basal type, i. e. inserted perpendicularly to the nanostructure length (Fig. 1 bottom). Thee optical properties of the nanostructure were studied by spectroscopy of the emitted light when it was exposed to short (~0.2 picoseconds) ultraviolet laser-pulses at low sample temperture (5 Kelvin).
It was revealed, that the nanostructures indeed demonstrate unusual light emission with clear components attributed to either the stacking faults for the wurtizte crystal (see Fig. 2 bottom). Light emission occurs only when two oppsitely charged particles, electrons and holes, share the same region of space. Since the electrons are confined in the zinc blende segment while the holes remain in the wurtzite crystal is the propability of light emission reduced for the stacking fault. In the spectrum this effect can be clearly seen as a long intensity decay time (~360 picoseconds) after the laser pulse, to be compared with the short decay time (~72 picoseconds) of the spectral feature attributed to electrons and holes both in the wurtzite crystal (see Fig. 2 bottom). These results confirm our idea that the stacking faults are segments of zinc blende that traps electron (but not holes).
To conclude, our study demonstrates the possibility for design of quantum wells based on single material via specific defects introduction. Thus, the role of stacking faults in zinc oxide is shifted from being negative, as a defect, to a convenient tool for tuning the electronic and optical properties of the material. This approach allows avoiding of many negative effects, expected on the interface of different materials, like stress and strain accumulation, species intermixture and segregation. Once the insertion of zinc blende into wurtizte is controlled, this alternative approach of material design can be considered for practical applications for the design of next-generation optoelectronic and photonic devices.
Details of the research are described in Nanotechnology 21, 215202 (2013)
- Linköping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM)
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Last updated: 02/11/14