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Comparison of the material properties of GaInN structures grown with ammonia and dimethylhydrazine as nitrogen precursors

Since group III nitrides and their ternary alloys have a direct band structure with band gaps between 0.9 and 6.2 eV when grown in the wurzite structure, these materials are most promising for optoelectronic applications in the short wavelength region. However, many critical issues still exist in the fabrication of these structures using metalorganic vapor phase epitaxy (MOVPE). In the case of GaInN with high In mole fractions, the difference in thermal stability between GaN and InN force us to do the growth at temperatures below 800°C [1]. At such low temperatures, the cracking of the commonly used nitrogen precursor ammonia (HN3) is kinetically limited [2], increasing drastically the necessary V/III-ratio not only to avoid the formation of In droplets but also to achieve a good crystalline and optical quality. The use of a nitrogen precursor that decomposes at lower temperatures is expected to result in a more efficient process and to be useful for the growth of less thermally stable alloys such as GaInN. Dimethylhydrazine (DMHy) is such a less stable liquid compound with a relatively high vapor pressure. It has been already used for the growth of hexagonal GaN [3] and for mixed group V alloys (GaInAsN) at low temperatures [4]. The main purpose of our current research is to grow GaInN-layers at low temperatures using this alternative nitride precursor. We applied low-pressure MOVPE using the standard precursors triethylgallium (TEGa), trimethylindium (TMIn) and trimethylaluminum (TMAl) as group III sources. On a (0001) oriented 6H-SiC-wafer, we grew a 800nm thick Al0.12Ga0.88N buffer layer, followed by a 1 µ ; ; ; ; ; m thick GaN layer at 1120°C. Finally, a GaInN quantum well was deposited, covered by another thin layer of GaN. The GaInN film was grown with NH3, DMHy or a mixture of both in order to study systematically the applicability of the latter nitrogen precursor. Moreover, we have studied the influence of different growth parameters such as the growth temperature and the growth rate. The characterization of these structures has been done by using photo- and cathodoluminescence. At growth temperatures of 800°C and more, we found a strong decrease of photoluminescence intensity for DMHy grown layers. Another critical point for the growth of GaInN is the tendency to develop strong composition fluctuations on a sub-micrometer scale owing to the thermodynamic immiscibility of GaN and InN for In concentrations exceeding 10%. This may be overcome or transformed into a benefit by pushing the growth of quantum dot structures. The growth of self-assembled quantum dots (QDs) via the Stranski-Krastanow growth mode has already been studied in many material systems showing excellent optoelectronic properties. In order to achieve self-assembling, a further reduction of the growth temperature is required which again favors the use of a less stable nitrogen precursor. Therefore, we have investigated the growth of GaInN self-assembled QDs on a GaN buffer layer via the Stranski-Krastanow growth mode. Up to now, we obtained a density of 1010 dots/cm2 at 720°C when using NH3, whereas a lower density of about 2.6x109 dots/cm2 has been achieved with DMHy. Moreover, the size distribution differs from that of the QDs grown with NH3, probably due to the different kinetic characteristics of the adducts liberated by the cracking of DMHy.

InGaN ; MOCVD ; semiconductors

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