Halide semiconductors stand at the very beginning of semiconductor science and technology. CuI was reported as the first transparent conductor, and the first field effect transistor was made from KBr. Although halogens are frequently used in semiconductor preparation, little use is currently made from halide semiconductors in electronics and photonics. We review past reports on the metal halide semiconductor CuI and related alloys and discuss recent progress with regard to this material including its use in organic electronics and solar cells as well as our own work on fully transparent bipolar heterostructure diodes (p‐CuI/n‐ZnO) with high rectification of several 107 and ideality factors down to 1.5. γ‐CuI(111) thin film on glass (1 × 1 cm2) and IV‐characteristics of p‐CuI/n‐ZnO/a‐Al2O3 bipolar heterojunction diode.

A multistep pulsed-laser deposition (PLD) process is presented for epitaxial, nominally undoped ZnO thin films of total thickness of 1 to 2 μm on c-plane sapphire substrates. We obtain reproducibly high electron mobilities from 115 up to 155 cm2/V s at 300 K in a narrow carrier concentration range from 2 to 5×1016 cm−3. The key issue of the multistep PLD process is the insertion of 30-nm-thin ZnO relaxation layers deposited at reduced substrate temperature. The high-mobility samples show atomically flat surface structure with grain size of about 0.5–1 μm, whereas the surfaces of low-mobility films consist of clearly resolved hexagonally faceted columnar grains of only 200-nm size, as shown by atomic force microscopy. Structurally optimized PLD ZnO thin films show narrow high-resolution x-ray diffraction peak widths of the ZnO(0002) ω- and 2Θ-scans as low as 151 and 43 arcsec, respectively, and narrow photoluminescence linewidths of donor-bound excitons of 1.7 meV at 2 K.

Transparent and electrically conducting p-type copper(I)-iodide thin-films form highly rectifying p-CuI/n-ZnO diodes. Sputtered copper thin films on glass were transformed into polycrystalline γ-CuI by exposing them to iodine vapor. The electrical parameters extracted from Hall effect are p=5×1018 cm−3, μh,Hall=6 cm2/Vs, and ρ=0.2 Ωcm for hole concentration, mobility, and electrical resistivity, respectively. Heterostructures consisting of p-CuI and pulsed-laser deposited n-ZnO were fabricated on a-plane sapphire substrates. The p-CuI/n-ZnO diode exhibits a current rectification ratio of 6×106 at ±2 V and an ideality factor of η=2.14.

The group‐III sesquioxides possess material properties that render them interesting for applications such as high‐power rectifiers and transistors, solar‐blind UV detectors and inter‐sub‐band infrared detectors. Technology for growing large, single‐crystalline bulk material and for wafer fabrication exists, enabling homoepitaxial growth of thin films with high crystalline quality. The bandgap can be tuned in an energy range from about 4 to 8 eV for the ternary alloys and allows growth of heterostructures with large band offset. Here, past results and recent investigations on the growth, the material properties, contact fabrication and the alloying of group‐III sesquioxides are reviewed, and an overview on demonstrator devices is provided.

Cuprous iodide has been investigated since 1907 when Karl Bädeker prepared this material from metallic copper thin films with subsequent iodization and reported it as fully transparent conductor. Nowadays CuI is recognized as p‐type wide bandgap, transparent semiconductor, offering rather high hole mobilities of so far up to 10 Vs∕cm2 in thin films. The charge carrier density is primarily controlled via the amount of copper vacancies. CuI has been prepared as bulk material and substrate and thin film as well as in the form of various nanostructures. Thin films can be prepared by various techniques including iodization of copper and by thermal evaporation, sputtering or pulsed laser deposition of CuI. Recent progress is represented by the epitaxy on other semiconductors, in particular zinc oxide. CuI has found use as intermediate layer between ITO and organic absorbers in solar cells. Recently, bipolar heterostructure diodes prepared from p‐CuI∕n‐ZnO layers on sapphire were found to exhibit very high rectification. This makes CuI interesting for use in transparent electronics. For further details see the Review Article by M. Grundmann et al. on pp. http://doi.wiley.com/10.1002/pssa.201329349.