Group-III nitrides

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The following text has been extracted from my PhD thesis. If you find any of this info useful, please make sure to reference it appropriately.

What are group-III nitrides

The group-III nitrides, or more correctly group-IIIB nitrides, comprise the compounds of nitrogen (N) and the elements in the first column of the p-element block of the periodic table, boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). The other group-III nitrides are compounds of N and the transition metals in group-IIIA, mainly scandium (Sc) and yttrium (Y). Other transition metals in group-IIIA are rather more exotic and include some radioactive elements: lanthanum (La), lutetium (Lu), actinium (Ac) and lawrencium (Lr). There are ongoing investigations on the properties of Sc-containing nitrides, that present interesting features in the context of functional materials research.[1][2][3][4][5][6] The most technologically important of these compounds are GaN, InN, AlN and their alloys. BN presents structural and electronic properties that do not follow the trends of GaN, InN and AlN, and is most interesting in solid-state physics for nanotube applications.[7][8] There is a possibility that TlN could be used alloyed with the other nitrides given some similarities in their electronic properties.[9] However, perhaps partly because Tl is extremely toxic and its use in industry or research raises health issues,[10] there exist very few studies on the properties and potential uses of TlN. Therefore, whenever we use the terminology group-III nitrides, we implicitly refer to the binaries GaN, InN and AlN, the ternaries InGaN, AlGaN and AlInN, and the quaternary AlInGaN.

Band gap Eg of the technologically important III-V binaries as a function of lattice parameter (in-plane lattice parameter in the case of WZ nitrides). Solid circles indicate a direct gap material while empty circles indicate an indirect gap. The curves show the band gap for some of the ternaries, where solid lines indicate direct gap and dashed lines indicate indirect gap. The data for the cubic III-Vs has been taken from Ref.[11], except for ZB nitrides, which is from Ref.[12]. The data for the WZ nitrides is from Wu's review paper.[13] The band gap of the ZB ternaries has been calculated as the minimum of the gaps at Γ, X and L from the respective band gap bowing parameters provided in Refs.[11][12], using a quadratic interpolation with bowing parameter. The variation of Eg with composition for the WZ nitride ternaries has been calculated using the same type of interpolation from the bowing parameters recommended by Wu.[13] A linear interpolation (Végard's law) is assumed for the lattice parameters of the ternaries. All data is for the zero-temperature limit T = 0.

The III-N's story

The use of nitrides as versatile semiconductor materials has driven significant advances over the last two decades in the field of optoelectronic applications. This is mainly due to the fact that the wurtzite (WZ) III-N materials are all direct band gap semiconductors. By alloying GaN (Eg = 3.43 eV), InN (Eg = 0.64 eV) and AlN (Eg = 6.14 eV) one gains potential access to the whole visible spectrum, as well as near infrared (IR) and ultraviolet (UV),[13] as we show in the figure in a direct comparison with other III-V materials. Although zinc-blende (ZB) nitrides might in principle appear as an alternative to their WZ counterparts, the truth is that growth of ZB nitrides is complicated and single-phase crystals are very difficult to achieve.[14] There is no other material system that allows light emission over such a wide spectral range, while at the same time maintaining a direct gap. GaN-based heterostructures have been successfully employed in the commercial fabrication of violet and blue light-emitting diodes (LEDs)[15][16] and laser diodes (LDs).[17] Other emerging applications of the nitrides include the extension of LEDs towards longer[18] and shorter[19] wavelengths, and their use in high electron mobility transistors (HEMTs)[20] and as multijunction high-efficiency solar cells.[21] Group-III nitrides emerged during the last decade of the twentieth century as a solution to the production of blue and green light. Shuji Nakamura, at Nichia Corporation in Japan, worked on the development of GaN-based light-emitting diode (LED) and laser diode (LD) technology from 1989 until 1999.[17] This allowed Nichia to produce and commercialize blue LEDs (1993), green LEDs (1995) and violet LDs (1999).[17]

Not all is well with the nitrides

However, and in spite of their evident success, there are still dark areas in the field of nitride research, and the booklet of properties of these materials is ever expanding. This is well illustrated by the fact that Vurgaftmann and Meyer published a revised version of their review, for the nitrides only,[12] scarcely two years after the publication of their almost-biblical anthology on band parameters of the III-Vs.[11] A quantity as fundamental as the energy gap of InN was revised between the two versions by more than a factor of two, from 1.99 eV to 0.78 eV. It seems that its zero-temperature value has been finally established at around 0.69 eV.[13] In this context, the (eventually enormous) success of the Nichia/Nakamura tandem is very striking: the most suitable substrate for GaN, sapphire (Al2O3), presents a lattice mismatch of 16%, meaning that GaN grown on sapphire exhibits a large dislocation density. Dislocation densities of the order of 109 cm-2 are typical in InGaN quantum wells (QWs),[22] with lower (but still relatively high) values achievable by techniques such as epitaxial lateral overgrowth (ELOG).[23] However, dislocations act, in principle, as non-radiative recombination centres and dislocation densities as low as 103 cm-2 are known to quench light emission in other III-Vs, such as GaAs.[16][24][18] To date, the reason or reasons behind this unexpectedly high efficiency of GaN-based devices remain a matter of debate. Some researchers have attributed it to the presence of alloy fluctuations, In-clustering and the associated local variations in the carrier confinement potential,[25][26] while others have suggested QW width fluctuations as the origin of the observed high efficiency.[24]

Other important issues regarding growth of nitrides with high In content (including InN itself) are strain and miscibility. High In content alloys are needed in order to extend device operation energies towards longer wavelengths. While the ternary AlGaN can be grown practically at any desired composition, there are severe limitations as to how much In can be incorporated into InGaN and AlInN alloys.[27][28] Nitrides are usually grown by either molecular beam epitaxy (MBE) or metal-organic vapour-phase epitaxy (MOVPE). MOVPE was the technique originally used by Nakamura at Nichia, and has since remained the method of choice because of advantages regarding device efficiency and growth rate, compared to MBE.[29][30]. More recently, plasma-assisted MBE seems to have taken a leap forward and is improving in quality upon more traditional MBE techniques.[30] InN and GaN have very different optimal growth temperatures, and InGaN alloys are grown on GaN templates, which means, given the lattice mismatch between InN and GaN (see the figure above), that the strain in the InGaN layer will significantly increase as the In molar fraction increases.

Electric polarization and built-in fields

One of the main problems that are encountered when dealing with nitride heterostructures, and one that severely affects device efficiency, is the existence of built-in electrostatic fields. Although the other III-V materials also present piezoelectric properties due to their cubic ZB structure, the WZ crystal structure in which III-N materials crystallize is compatible with the existence of both spontaneous and piezoelectric polarization. Additionally, the increased ionicity of the III-N compared to the other III-V accounts for a much stronger effect. The discontinuity of the polarization vector between two nitride materials that differ in composition and/or strain state leads to a strong polarization potential, with effects on the electronic properties of these systems.

The study of strain effects in III-N materials, including electric polarization in particular, was the central theme of my work as a PhD student in Cork. The first studies that we carried out were on polarization field control in nitride QWs and quantum dots (QDs). We proposed composition engineering as a route towards suppression (or reduction) of the strong built-in electrostatic fields present in III-N QWs and QDs. During the time in which that work was carried out, we encountered several aspects of the description of nitride materials that, in our opinion, needed improvement. Specifically, the description of the electric polarization in nitride alloys previously available was based on the implicit assumption that its value only depends on macroscopic strain and average composition in the sample. However, we challenged this simplified virtual crystal model and showed that the local atomic landscape matters and has a large impact on the computation of average properties, introducing a local polarization theory and showing how these local effects affect the electronic properties of InGaN.[31]


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