Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

III-V-based nanowires usually exhibit random mixtures of wurtzite (WZ) and zinc blende (ZB) crystal structure, and pure crystal phase wires represent the exception rather than the rule. In this work, the effective group V hydride flow was the only growth parameter which was changed during MOVPE growth to promote transitions from WZ to ZB and from ZB to WZ. Our technique works in the same way for all investigated III-Vs (GaP, GaAs, InP, and InAs), with low group V flow for WZ and high group V flow for ZB conditions. This strongly suggests a common underlying mechanism. It displays to our best knowledge the simplest changes of the growth condition to control the nanowire crystal structure. The inherent reduction of growth variables is a crucial requirement for the interpretation in the frame of existing understanding of polytypism in III-V nanowires. We show that the change in surface energetics of the vapor-liquid-solid system at the vapor-liquid and liquid-solid interface is likely to control the crystal structure in our nanowires.

Nanowires grown with the vapor-liquid-solid method commonly exhibit polytypism, showing both zincblende and wurtzite crystal structure. We have grown p-type InP nanowires using DEZn as a dopant precursor and studied the wetting of the seed particle and the nanowire crystal structure. The nanowires grown with high DEZn molar fractions exhibit deformed seed particles after growth. We observe 20% smaller nanowire diameter at the highest DEZn molar fraction, indicating a significant increase in contact angle of the seed particle during growth. The decrease in diameter correlates with an increase in zincblende segment length as measured by TEM. We explain the results with a modified nucleation model.

We report a method using in situ etching to decouple the axial from the radial nanowire growth pathway, independent of other growth parameters. Thereby a wide range of growth parameters can be explored to improve the nanowire properties without concern of tapering or excess structural defects formed during radial growth. We demonstrate the method using etching by HCl during InP nanowire growth. The improved crystal quality of etched nanowires is indicated by strongly enhanced photoluminescence as compared to reference nanowires obtained without etching.

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Semiconductor III-V nanowires are promising components of future electronic and optoelectronic devices, but they typically show a mixed wurtzite-zinc blende crystal structure. Here we show, theoretically and experimentally, that the crystal structure dominates the conductivity in such InP nanowires. Undoped devices show very low conductivities and mobilities. The zincblende segments are quantum wells orthogonal to the current path and our calculations indicate that an electron concentration of up to 4.6 × 10(18) cm(-3) can be trapped in these. The calculations also show that the room temperature conductivity is controlled by the longest zincblende segment, and that stochastic variations in this length lead to an order of magnitude variation in conductivity. The mobility shows an unexpected decrease for low doping levels, as well as an unusual temperature dependence that bear resemblance with polycrystalline semiconductors.

9Populärvetenskaplig sammanfattning 11List of papers 15 AbstractIn this thesis, in situ doping during growth of III-V semiconductor nanowires, primarily for photovoltaic applications, is investigated. The nanowires were grown by metalorganic vapor phase epitaxy (MOVPE), with gold seed particles. After growth the nanowires were characterized using various techniques, including photoluminescence, transmission electron microscopy and electrical measurements of contacted nanowires. Different III-V materials were studed, both binary materials such as InP and GaAs, and ternary materials such as Ga x In 1-x P. To achieve p-and n-doping, different precursors were employed.The results show that successful p-and n-doping can be achieved in many materials. The in situ doping is shown to affect the nanowire growth strongly, but differently depending on the combination of material and dopant. The main effects are related to the growth rate and the crystal structure. It is shown that the ndopant H 2 S increases the growth rate and induces wurtzite crystal structure in InP nanowires, while the p-dopant DEZn gives an unchanged growth rate with zinc blende crystal structure. En solcells förmåga att omvandla solljus till elektricitet kallas för verkningsgrad. Idag baseras de flesta solceller på grundämnet kisel, vilket är samma material som är basen för elektroniken i alla datorer. Detta material ger en medelmåttig verkningsgrad för ett medelmåttigt pris. För att göra solceller ännu mer användningsbara utvecklar många forskare och företag nya sorters solceller, som är billigare eller bättre än kiselceller. De mest effektiva solcellerna görs av så kallade III-V ("tre-fem") material, som kan ge ungefär dubbelt så mycket eleffekt som kiselceller. Tyvärr är III-V material dyra, vilket gör dessa solceller olönsamma för de flesta tillämpningar.Ett möjligt sätt att sänka kostnaderna är att göra III-V materialet i form av små, avlånga kristaller som kallas nanotrådar. Dessa nanotrådar är ungefär en tusendel så tjocka som ett hårstrå, vilket betyder att deras diameter är ungefär samma som ljusets våglängd. Mängden III-V material som används kan då minskas på två sätt. Dels kan man använda ett annat, billigare material som bärande substrat, dels behöver man bara täcka ungefär en tiondel av ytan. Nanotrådarna fungerar som små antenner som effektivt fångar in solljuset. I en solcell absorberas ljusenergin av elektroner, som tvingas i en bestämd riktning vilket skapar en elektrisk ström. För att göra detta används så kallad dopning, 13 vilket innebär att man stoppar in kontrollerade mängder av speciella föroreningar i den annars mycket rena kristallen. Med n-dopning skapar man ett överskott av elektroner, och med p-dopning skapar man ett underskott. Kombinerar man ett pdopat och ett n-dopat område får man en p-n övergång, som också kallas en diod. Det är i övergången mellan p-dopning och n-dopning som elektronerna tvingas i en speciell riktning, nämligen mot den n-dopade sidan. För att nanotrådarna ska fungera som solceller måste dä...

We have grown InP nanowires doped with hydrogen sulfide, which exhibit sulfur concentrations of up to 1.4%. The highest doped nanowires show a pure wurtzite crystal structure, in contrast to bulk InP which has the zinc blende structure. The nanowires display photoluminescence which is strongly blue shifted compared with the band gap, well into the visible range. We find evidence of a second conduction band minimum at the gamma point about 0.23 eV above the band edge, in excellent agreement with recent theoretical predictions. Electrical measurements show high conductivity and breakdown currents of 10(7) A/cm(2).