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Understanding and optimization of InN and high indium containing InGaN alloys by metal organic chemical vapor deposition

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  1. Tez No: 400945
  2. Yazar: ÖCAL TUNA
  3. Danışmanlar: PROF. DR. MICHAEL HEUKEN, PROF. DR. RAINER WASER
  4. Tez Türü: Doktora
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2013
  8. Dil: İngilizce
  9. Üniversite: Rheinisch-Westfälische Technische Hochschule Aachen
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Elektrik Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 165

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Özet (Çeviri)

The most drastic breakthrough for InN, which was the measurement of the real bandgap of 0.65 eV instead of the previously accepted bandgap values of 1.9 - 2.1 eV[34, 35, 36, 37], makes it an attractive material among other III-V semiconductors. At the same time, the revision in bandgap has made the InGaN alloys more signicant since one can cover the whole solar spectrum by only changing In composition in InGaN alloys. However, growing these materials in high quality is very challenging. Even though the growth of these materials by MBE yields better quality than MOCVD grown samples; MOCVD is the preferred growth technique from the view of mass production. Alloying of these materials with MOCVD grown of other nitrides, such as GaN and AlN, is essential to develop devices for instance InGaN based solar cells and AlInN based HEMT. With these motivations, the goal of this study was the growth of high-quality n- and p-typedoped InN and InGaN alloys with dierent In content and the detailed characterization. In chapter 5, the inuence of the most critical MOCVD growth parameters, temperature and V/III ratio, on InN was investigated in detail. A proper growth region with respect to the structural, optical and electrical properties of InN as well as prevention of In droplet formation on the surface of InN was obtained. High (565 oC) and low (500 oC) growth temperatures and low V/III ratio result in In droplet formation on the surface due to lack of reactive N atoms. By increasing the V/III ratio and by keeping the growth temperature in the middle range (520 oC), In droplet formation was prevented. InN with a low screw dislocation density of 4.5x108 cm??2 and a relatively high edge dislocation density of 1.0x1011 cm??2 were found. High lattice mismatch between InN and GaN is the main reason for high edge dislocation density. By Hall eect measurements, a carrier density of 4.1x1018 cm??3 with a high electron mobility of 1200 cm2/Vs was obtained from the InN layer grown at high temperature of 550 oC. A decrease in carrier concentration with rising growth temperature was observed. The decrease in the carrier concentration at high growth temperature was attributed to the decrease of the density of N vacancy. The decrease in the N vacancy concentration at high growth temperature has been proven by positron annihilation spectroscopy. In order to eliminate the inuence of the surface electron accumulation layer on the electrical characterization results of the InN layers, optical techniques were proposed as alternative methods for characterization of the electrical properties. For that, Raman mode with respect to longitudinal phononplasmon (LPP) was analyzed. It was shown that a InN layer, which was grown at the highest V/III ratio, yield a low carrier concentration in the range of 9.0x1017 cm??3 compared to 6.0x1018 cm??3 as determined by Hall eect measurements. The low carrier concentration value indicates the high quality of the InN layer. The comparison of the results from Raman spectroscopy and Hall measurements suggest that the Hall measurement are dominated by the surface accumulation layer and therefore, it is an inappropriate technique for characterization of electrical properties of InN. A large thermal stability dierence between InN and GaN, very high equilibrium vapor pressure of nitrogen over InN and temperature stability of NH3 hamper the growth of high quality In-rich InGaN layers, which are essential for multi junction solar cells as well as for red LEDs. In this study, a new approach was proposed, which is based on the enhancement of In incorporation by inserting a thin InN interlayer beneath the InGaN layer. As a result of this approach, an additional increase of In content in InGaN with an amount of 11% was achieved. For instance, while without the InN interlayer, the In content was about 74%, it increased to 85% when the InN interlayer was grown. The increase in In content is explained by In segregation from the InN interlayer to the InGaN layer. This explanation was reinforced by RBS measurement results. With the InN interlayer method, InGaN layers with various In contents ranging from 40% to 85% were successfully grown. This method can be complementary to growth temperature for increasing In content. Because with decreasing temperature, the In incorporation is enhanced but at the cost of less crystal quality of InGaN. Structural and optical characterization results showed that the InGaN layers in the middle In composition range have the lowest quality. This is attributed to a high degree of atomic disorder and the large atomic radius dierence between In and Ga atoms. Optical investigation conrmed the existence of a strong phase separation in the middle of the InGaN composition range. In other words, optical spectra of InGaN layers in the middle InGaN composition range exhibits more than one emission peak. This is believed to be originated from the formation of local minima in the band tail which is cause by In uctuations as well as inhomogeneous lattice deformation and high density of impurity states. By XPS and IR reectivity measurements, band bending and the related surface electron accumulation were observed in InGaN layers. In the light of these results, it could be concluded that the position of bulk Fermi level with respect to charge neutrality layer plays a very important role in surface electronic properties of semiconductors. For the investigated In-rich InxGa1??xN layers, a transition from an accumulation layer to a depletion layer at the surface is observed at a composition slightly less than XIn =0.20. p-Type InN and InGaN are essential in order to be used as hole injection layers for devices such as green laser diodes and long-wavelength emitters. In chapter 7, synthesis and a systematic study of structural, optical and electrical properties of Mg-doped InxGa1??xN layers were presented. In the rst part of chapter 7, it was demonstrated that p-type InGaN layers with about 18% In can be grown with an acceptor concentration of 3.5x1018 cm??3. Even with low annealing temperature (750 oC), p-type conductivity was achieved. This means that Mg activation energy in these materials is lower that the activation energy of Mg in Mg-doped GaN. Temperature dependent PL revealed the activation energy of Mg in In0:18Ga0:82N:Mg is about 80 meV which is much lower that the activation energy of Mg in Mg-doped GaN which is about 160 meV. Even though the growth and annealing temperature of the In0:18Ga0:82N:Mg are lower than in the case of Mg-doped GaN, these temperatures are still high for the growth of solar cells and red LED structures since the active layers of these structures must be grown at low temperature to enhance In incorporation in InGaN. Therefore, p-type In-rich InGaN layers are necessary. With this motivation, in the second part, Mg-doped Inrich InxGa1??xN (XIn: 0.30, 0.51, 0.79) layers were examined. The layers were grown at a low temperature of 550 oC on GaN templates. The samples were annealed at a low temperature of 565 oC in order to activate Mg atoms. ECV results indicated that p-type buried layers exist in all Mg-doped In-rich InGaN layers. As expected, a surface accumulation layer was observed. A strong correlation with the amount of surface accumulation and In content in InGaN was obtained. It was also shown that Mg doping concentration is critical for the conductivity type change. For instance, in the case of InN:Mg, p-type conductivity was obtained when the InN layers were doped with Mg concentration in the range of 2.6x1020 cm??3. Mg concentration higher than this value was observed to change the conductivity type of InN:Mg to completely n-type. In the case of over doping, Mg atoms form complexes with N vacancies which behave as donors and therefore results in a change in conductivity from p- to n-type. In conclusion, in terms of electrical properties such as low carrier concentration of about 9x1017 cm??3 and high electron mobility of 1200 cm??3 and optical properties of low energy bandgap and narrow line widths of PL emission peaks (less than 100 meV) conrm the achievement of the good quality of MOCVD grown InN layers. A comparison of these results to the literature is reinforced the good quality of the layers achieved in this study[44, 250, 251]. The employed optical techniques for the determination of bulk carrier concentration and guring out the reasons of the high bulk carrier concentration help to push forward the growth of good quality InN layers. For In-rich InGaN layers, the proposed method of growing InGaN layers directly on a thin InN interlayer is the key point for the achievement of the In-rich InGaN layers. In addition, as a rst time, for MOCVD grown In-rich InGaN layers, it was shown that the transition from an electron accumulation to an electron depletion occurs when the In content is about 20%. Many physical aspects behind the magnesium doping mechanism for MOCVD grown InN and various Indium containing InGaN alloys were deeply investigated. It was obtained that Mg doping concentration as well as In composition plays important roles for doping mechanism. Depending on the In content in InGaN, the required Mg doping concentration to achieve p-type conductivity was found to be dierent. For example, a high hole carrier concentration of 3.5x1018 cm3 was achieved for In0:18Ga0:82N layers by controlling the Mg doping concentration. Above a critical Mg doping concentration, Mg atoms form clusters with defects and therefore behave as donors in grown materials. Further investigations of electrical properties by employing variable led Hall eect and temperature dependent Hall eect measurements are need to be done in order to understand and improve the material properties.

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