Free-standing zinc-blende (cubic) GaN, AlN and AlGaN layers grown by molecular beam epitaxy

III族氮化物半导体（AlN、GaN和InN及其固溶体）越来越多地用于琥珀色、绿色、蓝色和白色发光二极管（LED）、用于蓝色/UV激光二极管（LD）以及用于高功率、高频和高温电子器件。然而，阻碍该领域进展的最严重问题之一是合适的基底的稀缺性。AlN、GaN、InN和它们的固溶体通常生长在非晶格匹配的衬底上，例如蓝宝石、GaAs或SiC，但是体GaN和AlN衬底对于最高质量的基于氮化物的器件会更好。对于用于紫外光电子学和高功率水平下的高频应用的AlGaN基器件，AlN衬底将是理想的。与GaN相比，AlN衬底具有更高的辐射硬度和上级热导率。GaN和AlN的晶格参数存在可测量的差异，因此对于几种器件应用，AlGaN衬底将优于GaN或AlN。立方AlN和AlGaN衬底的成功生产将意味着我们的技术可以在商业上扩展到水消毒，生物恐怖主义探测，卫星通信和数据存储设备。III族氮化物通常以六方（纤锌矿）结构结晶。与传统的III-V族半导体相比，纤锌矿III族氮化物的独特特征是晶体结构内部存在非常强的电场。由于电荷分离，这些降低了量子威尔斯中的光发射强度。纤锌矿材料中的电场可以通过非极性方向生长来消除。然而，消除电场的直接方法是使用非极性（100）取向的锌-立方（立方）III-氮化物层。到目前为止，亚稳态立方GaN和AlN层受到的关注比更熟悉的六方膜少。然而，由于三个主要原因，现在对锌-立方氮化物的兴趣迅速增加：1）立方（100）氮化物中不存在电场; 2）在垂直解理面中解理立方（100）氮化物的能力;以及3）载流子（特别是p型）的增强的迁移率。最近，我们首次证明可以通过等离子体辅助分子束外延（PA-MBE）生长独立的闪锌矿GaN层，并具有作为衬底的潜在应用。我们不知道任何数据或出版物的最新独立的锌层氮化铝或AlGaN层。本计画的主要目的是研究利用分子束外延法生长独立式锌立方氮化铝及氮化铝镓层的可行性，并全面分析其结构、光学及输运性质。这是开发商业上可行的立方氮化物衬底的第一步。

The group III-nitride semiconductors (AlN, GaN and InN and their solid solutions) are being increasingly used for amber, green, blue and white light emitting diodes (LEDs), for blue/UV laser diodes (LDs) and for high-power, high-frequency and high temperature electronic devices. However, one of the most severe problems hindering progress in this field is the rarity of suitable substrates. AlN, GaN, InN and their solid solutions are commonly grown on non-lattice matched substrates e.g. sapphire, GaAs or SiC, but bulk GaN and AlN substrates would be much better for the highest-quality nitride-based devices. For AlGaN-based devices for ultra-violet optoelectronics and for high frequency applications at high power levels, AlN substrates would be ideal. AlN substrates have higher radiation hardness and superior thermal conductivity compared to GaN. There is a measurable difference in the lattice parameters of GaN and AlN, therefore for several device applications AlGaN substrates would be preferable to either GaN or AlN. Success in producing cubic AlN and AlGaN substrates would mean our technology could be extended commercially to water sterilization, bioterrorism detection, satellite communication and data storage devices.The group III-nitrides normally crystallise in the hexagonal (wurtzite) structure. The unique feature of wurtzite group III-nitrides, in comparison with conventional III-V semiconductors, is the existence of very strong electric fields inside the crystal structure. These reduce the optical emission intensity in quantum wells, due to charge separation. The electric fields can be eliminated in wurtzite material by growing in non-polar directions. However, a direct way to eliminate electric fields would be to use non-polar (100) oriented zinc-blende (cubic) III-nitride layers. The thermodynamically metastable cubic GaN and AlN layers have, so far, received less attention than the more familiar hexagonal films. However, interest in zinc-blende nitrides is now rapidly increasing for three main reasons: 1) the absence of electric fields in cubic (100) nitrides; 2) the ability to cleave cubic (100) nitrides in the perpendicular cleavage planes and 3) the enhanced mobility of the carriers (particularly p-type). Recently we have demonstrated for the first time that it is possible to grow free-standing zinc-blende GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE) with potential applications as substrates. We are not aware of any data or publications to-date on free-standing zinc-blende AlN or AlGaN layers. The main aims of this project are feasibility studies of the growth of free-standing zinc-blende (cubic) AlN and AlGaN layers by PA-MBE and a comprehensive analysis of their structural, optical and transport properties. This is the first step towards developing commercially viable cubic nitride substrates.