The InGaN alloy system offers a unique opportunity to develop high efficiency multi‐junction solar cells. In this study, single junction solar cells made of Inx Ga1–x N are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si‐doped InGaN layer, an intrinsic layer and an Mg‐doped InGaN layer on the top. The I –V curves indicate that the cell made of all‐GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo‐responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The electrical properties of InGaN that is either undoped, or Mg doped, are compared to learn about the nature of p‐type conductivity. For In alloy fraction beyond 5% Hall measurements do not indicate p‐type polarity, even when Mg doping is employed. In contrast, hot probe measurements show that p‐polarity can be measured for the entire range of Mg‐doped In mole fractions. The conflicting polarity indications are primarily the result of surface electron accumulation. Parasitic surface electron conductivity can further be seen in p–n homojunctions at In fractions including 20% and 30%. This has an impact on structures such as solar cells. Temperature variable conductivity and PL from different layer structures provides further understanding of the nature of InGaN:Mg. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the growth of high quality Mg‐doped InGaN and InAlN alloys on (0001) sapphire substrates by molecular beam epitaxy. Phase separation does not occur in InxGa1‐xN films with Indium content up to 0.88. Hall measurement shows that a hole concentration of 7.7x1017 cm–3 is achieved on Mg‐doped In0.04Ga0.96N. When x > 0.11, the Hall samples exhibit strong n‐type polarity, whereas p‐type polarity is confirmed by hot probe measurement for all of Mg‐doped InGaN. Ni/Au film is deposited on Mg‐doped InxGa1‐xN as the p contact met al. A contact resistivity as high as 9.95 Ωcm2 is obtained when x = 0.25 while it is only 6.81x10–5 Ωcm2 when x = 0.85. High quality crack‐free InAlN films are also grown on sapphire substrates with AlN/InN quantum‐well buffer layers. When Mg is incorporated near the solid solubility limit, donor defects form and In0.62Al0.38N films show n‐type polarity by Hall measurement, however lower Mg flux yields p‐type polarity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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GaN p-i-n solar PV structures grown by rf plasma assisted molecular beam epitaxy (MBE) produce high performance IV characteristics with a leakage current density of less than 1×10−4 mA cm−2 at 0.1 V forward bias and an on-resistance of 0.039 Ω cm2. Leakage current measurements taken for different size diodes processed on the same sample containing the solar cells reveal that current density increases with diode area, indicating that leakage is not a large function of surface leakage along the mesa. Nonannealed Pt/Au Ohmic p-contacts produce a contact resistivity of 4.91×10−4 Ω cm−2 for thin Mg doped contact layers with sheet resistivity of 62196 Ω/◻. Under concentrated sunlight the cells produce an open-circuit voltage of 2.5 V and short circuit currents as high as 30 mA cm−2. Multiple growths comprised the study and on each wafer the IV curves representing several diodes showed considerable variation in parasitic leakage current density at low voltages on some wafers and practically no variation on others. It appears that a smaller grain size within the GaN thin film accounts for higher levels of dark current.